Australia raised 5 infantry divisions during the war, as well as other units such as the Australian Light Horse, and contributed it's navy to British efforts in the Pacific most notably.

There were two ANZAC Corps raised by the end of the war, typically containing three Australian and two Australian and the New Zealand division respectively. William 'Birdy' Birdwood, a British general, lead the ANZACs at Gallipoli, and then the Somme and Third Ypres. In c. May, 1918, he was given command of the British Fifth Army, leading to Australian John Monash being made commander of I ANZAC Corps. General Godley, an Australian, commanded the other corps.

Both corps gave good accounts of themselves, displaying great skill in the hundred days campaign at the end, but their achievements were often at a high price, and ought to be viewed in the wider context of the truly remarkable transformation that the BEF underwent during and after the Somme Campaign of 1916.

I agree with /u/JosephRMcCarthy that the ANZAC Myth, Charles Bean's most enduring legacy, has been blown completely out of proportion. However, the Australians were noted as displaying good morale and fighting spirit, though the latter often resulted in recklessness. They were also noted for being somewhat delinquent, making up 1/4 of the BEF's deserters, and were notoriously ill-disciplined.

All the same, Field Marshall Douglas Haig thought very highly of the men of the ANZAC.

Oh boy! Now I don't have my sources on me, but I am currently in final year B.A History at Murdoch university, and have done a bit of work on Australian involvement in WWI, both through course work and my spare time.

Australian contributions to the war in terms of manpower are of course overshadowed by France, Britain and the USA, entirely due to population. In regards to the campaigns they did fight in, most notably being Gallipoli they nonetheless did contribute a reasonably significant part of the force: I believe it to be approximately one third of the 15 or so divisions that took part in the campaign.

As for the Battle of Passchendale (Third Battle of Ypres), from my notes, I believe there was 3 Australian divisions taking part in the action, out of a total of 9 divisions

Was the contribution significant? Perhaps in regards to the size of Australia, population wise, certainly. However, Australians were largely under the control of British generals. While I mean no disrespect towards the Australians who fought in the war, the myth of the ANZAC, has in my opinion, certainly been blown well out of proportion.

While contributions were made, and they surely did effect Australia, one of the most important things to keep in mind is that Australia itself was, and still is, an incredibly young country, with no martial history to speak of. As such, what is commonly regarded to as a 'baptism of fire' (a term I personally find repulsive), was quickly seized as an identity for the new Australian federation.

Will attempt to gather my sources, by tomorrow AM, I hope this semi-answers you question in the meantime

The contribution of Australian forces in the Great War is discussed here as well. I would consider their contribution to be significant given their small population, as well as the qualitative performance of their troops once overseas.