It’s a biased opinion. I had the pleasure of studying under a classmate of Roland Perry, himself a leading Australian historian, and when I spoke to him once about Perry’s biography of Monash, my professor remarked that Perry had a tendency to present facts as though he had found them before anyone else had.

I haven’t read his work on Monash and Chauvel but from what I know of Perry’s writing but it sounds as though he is presenting Monash as the architect of the overall Allied victory, an opinion that he has presented before and not necessarily an uncommon one among popular historians. Local heroes sell books.

The battle itself is notable in Australian history as it does represent the first time that all five Australian divisions served alongside each other on the Western Front and under Australian command. It is also notable for the fact that Monash took a leading role in the planning of the attack which was modelled on the tactics Monash had successfully deployed at Le Hamel a month earlier. However Monash himself was under the command of Henry Rawlinson, the commander of the Fourth Army and Monash was joined by Arthur Currie, Richard Butler and Charles Kavanagh, Monash's fellow corps commanders. All of these men plus Rawlinson had responsibility for the planning and execution of the attack so while Monash does deserve great praise for the success of the attack, that praise needs to be spread among the other corps commanders (though Butler’s role has been minimised, especially in Australian works).

Now, to your question. The problem with the assertion that, had the Australians and Canadians kept advancing they could have destroyed a reserve army, is that it ignores the reality that Rawlinson's Fourth Army faced after the successes of August 8, 9 and 10. By the end of August 11, the Allied offensive had brought them to the border of the 1916 Somme battlefield. This area was crisscrossed with old trenches and was impenetrable to British cavalry which had performed incredibly well over the past few days chasing down and harassing Germans in open country. The infantry also had outrun its artillery and there were virtually no tanks left to offer support. The Germans, on the other hand, had stiffened their resistance and reserves were beginning to flood in, probably from that reserve army Perry asserts could have been destroyed. It was the perfect terrain for German delaying tactics and the attacking infantry, having already suffered large numbers of casualties needed time to rest, consolidate the ground they had taken and bring reserves in to replenish their depleted forces (it is also worth noting here that the Australia Corps had little to nothing in the way of reinforcements as voluntary enlistment had all but dried up and the depots were empty).

Haig meanwhile, although under pressure from Foch to continue the attack, was well aware of these issues and had learned from the past that now was the time to launch an offensive elsewhere and draw the German reserves away then continue the Fourth Army's advance. To that end, the British Third Army under General Julian Byng launched their own offensive north of the Fourth Army. Their objective was Bapaume. The Third Army attack was launched on August 21. In the meantime, the Fourth Amry was ordered to maintain contact with the enemy and prepare for a resumption of the advance which occurred on August 22.

Monash once remarked that the German defences were akin to a coiled spring, the further it was pushed back by advancing infantry, the greater the force of the counterattack as the distance between the frontline and the German interior lines of communication decreased. By pausing the Allied advance when Haig did, he avoided the full force of a German counterattack and by attacking to the north, he drew the German reserves away before resuming the Fourth Army's advance.

Source: Official History of Australia in the War of 1914-18 - Volume VI by Charles Bean