In World War 1, why were the Australian and Canadian troops so good?

It might also be worth making the points that:

a) The majority of Canadian troops were in any case recent British immigrants (estimated to be >70% in the early part of the war and >60 % in the latter part). A large proportion of these were fairly recent immigrants who had only been in Canada for < 8 years. A higher proportion of the officers were Canadian born because they were the doctors, solicitors, teachers, etc in pre-war towns.

b) The second point is that the cream of the British army were the regular forces (soldiers who were in the army before the war broke out). These soldiers, who were well trained, well fed (and consequently healthy and of good stature), and well equipped were killed or injured at battles such as Mons, Le Cateau, First battle of the Marne, First Battle of the Aisne, and First Ypres (and Gallipoli). The vast losses from these early battles were then replaced by the ill fed, poorly trained, poorly educated masses from Britain's industrial slums.

Nevertheless, many of these unheralded formations in Kitchener's army did extremely well (once trained sufficiently) including the 46th North Midland Division (North Staffs) who broke the Hindenburg Line in 1918 when other, more vaunted divisions, had failed.

In the case of Australia, WW 1 was the first major war with significant contribution of Australia towards the war effort of the Empire. Prior to WW 1, Australia contributed troops to the Boer War and the Boxer Rebellion Campaign.

So, on the part of the Australians we can expect a significant degree of propaganda highlighting the Australian contributions to WW 1. Especially, since the battlefields of Gallipoli and the Western Front were so remote from mainland Australia. Thus, painting the ANZAC's as especially good troops serves to maintain morale and support at the homefront.

Sorry, for the large amount of conjecture and lack of sources. See this as more of a thought or comment, rather than a full answer to the original question.

One can make much of the prairie and frontier background of the Canadian troops - and one should, as they were 8 cm (3") taller, 5'7" (178cm) at age 21 compared to just 5'4" (170cm) for Brits of the same era - but in no small part the success of the Canadian Corps post-Somme must be credited to its commander, Sir Julian Byng, being likely the most competent and innovative corps commander in either the French or British army.

Amongst other qualities, it was the dogged effort of Byng, his successor-to-be Sir Arthur Currie, and their staff that solved the conundrum of perfecting a walking barrage; allowing Canadian troops assaulting Vimy Ridge to be in German trenches reliably just 30 to 45 seconds after the barrage lifted. And in contrast to British regiments where only captains and above received artillery schedules, Byng and Currie issued schedules to every NCO down to corporal, as a means of guaranteeing that no troops got left behind due to officer casualties.

Pay the price of victory in shells - not lives. -- General Sir Arthur William Currie (1875-1933)

Pierre Berton's excellent book Vimy has a couple of early chapters on the symbiosis between Byng and his Canadian troops that resulted in such success on the battlefield.

The Australian historian and journalist LA Carlyon in his book Gallipoli reports Australian troops, a higher proportion of whom at that date had grown up in a rural, outdoor life (the same was probably true of Canadians), noticed that British troops who had grown up in the then smoky and crowded industrial cities of Britain often seemed less well nourished and healthy, even stunted in intellect.

This is not to say that the Australians were without faults. Vera Brittain who wrote about her experiences as a Nurse in World War 1 in 'Testament of Youth' said that if there was any trouble Australian troops were always ready to be part of it.

British War Correspondent Philip Gibb in his book 'Now it Can be Told' said that of British Empire troops the Scots and the Australians were more likely than others not to take prisoners but to kill enemy troops who had surrendered.

Britain recruited a higher proportion of its adult male population into the armed forces during the war, including by conscription from 1916, which meant that the British army probably came closer to 'scraping the bottom of the barrel' and taking some less suitable recruits. Australian, New Zealand and most Canadian soldiers were volunteers, hence more likely to be motivated.

Of course we should be cautious about accepting every generalisation made then or now about the qualities of soldiers of different nations, which can be biased by patriotism, stereotypes or chance as to e.g. which Canadian soldiers someone met and under what circumstances, as they were all individuals and doubtless some braver, more intelligent, better trained etc. than others.