Maybe higher rates of urbanization versus suburbanization? City people tend to vote left.
Australia is also one of the most urbanized countries on Earth (the top 10 Australian cities occupy about 70% of Australian populations). Yet even in its big cities conservatives do remarkably well. Take Sydney for example. It has been commented by Australians and Canadians alike that Melbourne is remarkably similar to both Toronto and Montreal politically. Paddy McGuinness has written this on Sydney Morning Herald on 20 September 2001 on the comparison between NZ and Australia, using Melbourne and Sydney as examples (unfortunately the article is gone from the net):
... The differences between the populations of Australia and New Zealand are about as different as those between Sydney and Melbourne.
That comparison does point to an interesting difference. There is a harder edge, a tougher minded approach to many issues in Sydney compared with Melbourne. There you tend to get more of the politics of the warm inner glow, along with pretensions to intellectual, cultural and moral superiority. Typically, the Fabian Society - wishy-washy socialism for idealistic and ineffectual intellectuals - flourishes there while it never did in Sydney. There is a similar difference between Sydney and New Zealand. Wellington is rather like a Canberra recruited entirely from the ranks of Melbourne school teachers. Not surprisingly, the NZ economy is in trouble when such people are in government.
There are real differences cross-Tasman, of course. The ethnic origins have always been rather different, with Catholic Irish much more heavily represented in Australia. We had a higher intake of non-English-speaking-background European migrants over the past 50 years. And the Maoris (and, increasingly, Pacific Islanders) are a much more significant proportion of the population than are Aborigines (however defined) here.
The do-gooders in New Zealand are, as in Melbourne, much more determined to get things wrong so they will feel good inside than are the majority of Australians, so they have set up disastrous institutions like the Waitangi Tribunal which fritter money away on compensation for past wrongs in such a way as to make the present day situation even worse. They are still strongly affected by old-fashioned pacifism, so they think that they need not spend money on their own defence.
As you can see, Sydney is actually moderate conservative by urgan Canadian (or even American) standards. Australian friends tell me that the conservatives there (Liberals) routinely gobble up all electorate seats situated at Sydney's North Shore (that's about 1/2 of the whole Sydney) and they also get some seats at Melbourne. Add to it the ALP (main party on the left) is actually to the right of Canada's Liberal Party and it amazes me to see Canada's politics is like that.
To help me judge the urban-vs.-suburban hypothesis: Does Sydney have more driveable and spacious surburbs, and Melbourne more walkable and densely packed city neighbourhoods, or are they about the same that way?
The other difference between Canada/NZ and US/Australia might be that the frontier played a much larger role in both the mythology and fact of US/Australian national life. Fighting nature leads to a more independent strain of mind; maybe it's no coincidence that the most conservative parts of Canada, Alberta and the BC interior, were our frontiers.