I am loathed to use Les Carlyon as a reference when answering questions here as his status within the history community is one of popular historian but I don't think I've seen an answer to your question put as succinctly as Carlyon put it in The Great War. I can't remember the exact quote and I couldn't find it in his 900 word tome of a book but it goes something like this:

Australians would never again face the main enemy on the main front and play as critical a role as they did during the First World War.

This is paraphrased but you get the idea. During the First World War, Australians made up about 10% of the British forces on the Western Front. They held large portions of the line and spearheaded many of the large attacks. The nature of the war made it relatively easy to inflict large losses on attacking infantry and although the Australians were often successful in their attacks, they still sustained large losses. From 1916 till the end of the war, their wasn't a single major offensive that the Australian Imperial Force wasn't involved in, the Somme, Arras, Third Ypres, The German Spring Offensive, the Hundred Days etc. These were among the largest, costliest offensives of the war for all involved and the Australians were for the most part, in the absolute thick of it.

Compare that to the Second World War. The Second Australian Imperial Force was supposed to head to Europe and fight the Germans as the First AIF had done but France fell before that could happen. Instead, they were deployed to North Africa where their role wasn't small but the mobile nature of that conflict (with a few exceptions) meant that the fighting their wasn't the meat grinder that the father's of the men fighting there faced on the Western Front some 24 years earlier. Additionally, the disasters in Greece and Crete were more costly in terms of the number of men taken prisoner then were killed. The same goes for the fighting in the Pacific where the Australians fighting in Singapore, New Guinea, Rabaul etc only faced a very small, albeit effective, Japanese force while the majority of Japan's army was tied up in China.

Reading back over this, it could be perceived that I am belittling the efforts and achievements of the Australian fighting men that served during the Second World War. That is in no way my intention but no one can argue that the battles fought by the men of the Second AIF were in anyway comparable to the size of battles fought in France and Belgium during the First World War or even on the Eastern Front during the Second World War.