The Australians provided most of the defending force for most of the First Battle of Tobruk (early April to late November, 1941). The initial defending force consisted of 4 Australian brigades (the 9th Australian Division and 1 brigade of the 7th Australian Division). Supporting the Australians were British artillery (including anti-tank and anti-aircraft units) and armour, and Indian troops. The commander was Lieutenant General Leslie Morshead, the Australian commander of the 9th Australian Division. Of the approximately 25,000 defenders, about 14,000 were Australian.

In August and September, most of the Australians were withdrawn, replaced by a Polish brigade, a Czechoslovak battalion, and a British division.

Simply due to their numbers, the Australians made an essential contribution to the battle, stopping German and Italian attacks, and counter-attacking and raiding. The Australians suffered over half the Allied casualties, about 3,200 of a total of about 6,000 - their reduced combat effectiveness due to the months of hard fighting was why the Australians were withdrawn. The British 70th Division, which was the main component of the troops which replaced the Australians, also fought hard, with over 2,000 casualties despite much less time spent in the battle.

As for the overall Australian contribution in North Africa, the initial Allied forces were small, about 36,000 Allied troops (1 British armoured division, 1 New Zealand division, and 1 Indian division, 14 more British batallions, and supporting troops) facing about 500,000 Italians, divided about equally between Libya and Italian East Africa. The Western Desert Force, facing the Italian on the Libyan front, consisted of 2 divisions (the British armoured division and the Indian division).

With small forces like these, even a modest Australian contribution was significant. The first major Australian land forces in the theatre was the 6th Australian Division, replacing the 4th Indian Infantry Division. This made the Australians the main infantry component of the Western Desert Force, and they played a key role in the defeat of the Italians in Libya, exemplified by their capture of Bardia, against a force of defenders almost three times their number, taking over twice their number prisoner.

The 6th Australian Division was joined by the 7th and 9th, and the Australians continued to play a key role against the rejuvenated Italians and their German allies (e.g., at the First Battle of Tobruk). When Japan expanded their war to include the British and allies, Australian troops were needed closer to home, and the 6th and 7th Divisions went home after Tobruk. The 9th Division stayed longer, taking part in the Second Battle of El Alamein. By then, the Allied forces, now the Eighth Army under Montgomery, had grown substantially, and the Australians were only about 10% of the Allied force. At the beginning of 1943, the 9th began its journey home.

New Zealanders stayed, and continued to fight to the end of the North African campaign, and then went on to fight in Italy.

The Australians certainly made an important contribution to the fighting in Africa, at a time when the British were stretched thinly. The crushing defeat of the Italians diverted German strength to Africa, and helped drain the German air force and supplying forces in North Africa was very costly to Axis shipping. However, other Allied forces, including the British, Indians, and New Zealanders, and others, also fought well. Success was not universal, and the Allies suffered defeats (such as the Second Battle of Tobruk in 1942, when Tobruk was lost in 4 days rather than defended for 8 months as in the First Battle of Tobruk). The Australians were not universally successful either, as their less-than-successful diversion to Greece showed.

Chest-beating pro-Australians will point at Australian success at Bardia and Tobruk and often over-emphasise Australian fighting effectiveness and the Australian contribution. But while the Australians generally fought well, they weren't super-soldiers - the key Australian contribution was through being there when Allied forces were scarce. When Allied forces were more plentiful, and the Australians went home to fight Japan, the remaining Allied forces went on to win a crushing victory in Africa (more Germans surrendered at the end of the fighting in Africa than at Stalingrad) without the Australians (but with the New Zealanders still with them).

So, I'm not sure if you're asking how significant was Australia's contribution to the defence of Tobruk, or are you asking how significant the Australian contribution to the defence of Tobruk was to the campaign or war as a whole?

I'm going to try and tackle the first two out of three for you there... because the third potential bit? Well, North Africa was by no means the largest or bloodiest campaign in the war even before we get into arguments about which was "most important" (a rabbit hole we might not want to go down) and it's hard to quantify what the impacts would've been if the Allies had not successfully prosecuted it, given quantifying hypotheticals comes with academic challenges.

In terms of the significance of Australian contribution to actions in and around Tobruk, the 6th Australian Division took Tobruk away from the Axis in the first instance over 21-22 January 1941, netting the majority of the 25,000 strong (mainly Italian) garrison along with a very significant haul of supplies and equipment. In the following months (as the campaign saw a couple of times) fortunes had reversed, and Tobruk basically stood in the direct path of Erwin Rommel's March-April counteroffensive east toward Egypt, and Rommel promptly did what he could to bypass and isolate it.

General Sir Archibald Wavell, as CINC Middle East, had assigned the relatively newly formed Australian 9th Division to the town's defence - although by no means should the popular perception of the Tobruk garrison as an Australian affair be too readily entertained. The support units for the town's garrison were overwhelmingly British and four regiments of Royal Artillery (so that's Brits, not Aussies) were present that would remain key to the defence of the town throughout the siege.

The other thing to remember in spite of the usual "Anzackery" (as Peter Stanley calls it) is the sad fact is that Major General Leslie Morhead's four Australian infantry brigades (20th, 24th and 26th from the 9th Division plus 18th Brigade from 7th Division) were in a pretty horrendous readiness state.

The 9th Division brigades were relatively inexperienced, they were lacking significant quantities of equipment against what was supposed to be assigned to them (an issue that most of the AIF Divisions at that stage of the war suffered from, with some formations often being "brought up to par" by basically depriving other formations of their equipment in lieu), and their level of individual training was both deficient in quantity (so, how much training time had been provided to individual soldiers and within formations) and scope (so, exercising at coordinating the different formation levels within the Division).

That said, the role assigned was a static and defensive one that helped reduce the impact of these deficiencies, and the Australians were able to play a significant role in Tobruk's defence, mainly through effective use of captured anti-tank artillery, resilience in holding and maintaining pre-existing Italian-built defence lines, and aggression in patrolling and raiding between the lines at night that minimised Axis opportunities to take the initiative, although all of these types of activities came at a significant cost in casualties (approximately 3000, with 940 taken prisoner).

Additional efforts by the RAN (and notably the sacrifices of the HMA ships Waterhen and Parramatta) further lent to the significance of this contribution to the first siege of Tobruk.

The second siege that occurred in January of 1942 had comparatively very little Australian involvement which was probably just as well - considering the efficacy of the Axis assault that took the town, if the 9th had been there the Australian government would've been looking at having three AIF divisions out of effective action by February 1942 (the 6th had been gutted in Greece and Crete in May 1941, prompting the Australian government to demand in July 1941 that the 9th Division be relieved from Tobruk over the next month or so, and February 1942 would see the loss of the bulk of the 8th at Singapore), to say nothing of the hypothetical impact to Eighth Army of not having an experienced and effective division that subsequently performed well at both El Alamein battles.

So the Australian contribution to the Siege of Tobruk was significant indeed. How significant was Tobruk generally to the North African campaign?

Well it did hamper Rommel's 1941 offensive in a year that was otherwise a pretty dim one for the Allies in immediately surrounding theatres, and there's a morale impact that goes along with that.

More practically, having a Division plus Change along a major supply route in your rear is never ideal, which is a situation that Rommel had to contend with, and the garrison managed to successfully tie up and attrit Axis troops that could've been used elsewhere in that theatre - arguably with these two points in mind, Tobruk's impact to the prosecution of the campaign was a positive one.

More parochially, the 9th Division was able to pick up valuable and hard (but not ruinous) combat experience that put it in good stead for some of the taxing engagements that would follow later in the campaign.

In terms of military influence Australians had on the campaign generally, RAN ships and RAAF personnel (the latter in very much a piecemeal fashion throughout the RAF as a, from an Australian viewpoint, ruinous result of the Empire Air Training Scheme) were active in the Mediterranean over the course of the North African campaign (and bordering ones) even before the Italians had declared war.

The RAN provided ships to multiple RN units, and the balance of three AIF divisions of increasing proficiency were militarily influential enough that the British would quite happily employ them in North Africa, Syria and Greece - so happily that in the case of the latter that Australian political and military leadership found itself seething in the first half of 1941 at a British government that was actively deceiving it in the hopes of simply utilising Australian military assets and personnel as the British saw fit (although Jeffrey Grey makes the argument that if both Australian Prime Minister Robert Menzies and then Lieutenant General Thomas Blamey had done their jobs properly, then the British wouldn't have had the chance to act as arbitrarily as easily).

Whilst it'd be easy to mount the argument with the evidence at hand that the British were keen to use any and every useful asset they could get their hands on in prosecuting campaigns in and around the Med (including North Africa) this logically goes with it that the Australian contribution to the campaign was well regarded - although that regard resultant from the military influence of Australian contributions rarely translated into any political influence over activities in the area, as seen in numerous examples of political tension between British and Australian leadership regarding deployment of Australians in Syria, Greece and even attempts to send Australians to Myanmar when Churchill and Curtin famously stouched over bringing the 9th Division back to Australia for action against the Japanese.

Whilst most of the citations about Australians that attributed to figures such as Rommel and Montgomery of annoying frequency are almost exclusively apocryphal, exaggerated or just plain made up by parochial old diggers down the RSL, Australian contributions to the campaign generally are considered to be both significant and effective (although by no means outstanding compared to other Commonwealth or Allied contributions) in that they assisted in an ultimately effective prosecution of a campaign that was considered key to the security of British supply lines to India and Asia.

SOURCES:

Peter Dennis, Jeffrey Grey, Ewan Morris, Jean Prior, Jean Bou - The Oxford Companion to Military History 2nd Ed (2008)

Jeffrey Grey - A Military History of Australia 3rd Ed (2008)

Wesley Olsen - Battalion Into Battle: The History of the 2/11th Australian Infantry Battalion, 1939-1945 (2011)

Mark Johnston - An Australian Band of Brothers: Don Company, Second 43rd Battalion, 9th Division (2018)