One of the problems of your counterfactual is that there were no other meaningful formations to fill in for the Australians. And a normal line unit would not have been a perfect substitute - as Sheffield and Terraine note the Australians were an elite unit. It is also noted that Australian divisions were 7000 the same as the British, but smaller than the Canadians which were 12000.That depends on your definition of important. Put it this way: if you took the Australian Corps out of the Amiens attack (keeping in mind it was one of three corps) and replaced it with another, would it still have succeeded? Yes, of course - perhaps not with the same level of success, but that would have as much to do with the fact any replacement corps would have less troops because Australian divisions were larger than their British counterparts and there were five of them in the corps. If Billy Hughes had chucked a tantrum and pulled the AIF in July 1918, would the Allies still have forced an armistice by November 1918? Yes.
Australians can be justifiably proud that in the space of four years a nation with virtually no professional army to speak of had raised and trained a (proportionally) large force that was, by 1918, one of the elite formations in the war. Monash was a superb general who lead his men well and possessed a great understanding of the nature of combat in the trenches. That should be enough - but for some reason people feel the need to one-up it and overstate the importance of the Australians on the Western Front.
The Australians neither won the war themselves nor stopped the Germans single-handedly, but they played an important role in the British line, as did the other Dominion forces.