We chose to move in September. Logic was I had a bonus paid that month, it is half way through the UK tax year so I get a rebate in the UK, and also in Australia I start work partway through the year and get a rebate as well. This is due to tax free allowances and tax bands applying for the full year even if you only earn for part of the year. Benefit depends on your income level, but it all helps.
Weather in Perth was nice Spring, not too hot and improving - got us all used to the heat gradually, not arriving in Jan to full 40 degree heat. We also got to enjoy UK summer and skipped a winter
As Quoll said the kids got a couple of months of school pre Christmas break which means they and we had made some friends before the 6 weeks holiday.
Work wise it took me 4 months to secure a role, but I was looking at a senior level where they are not that many around. Patience paid off.