I think the UK developer market gets skewed by contracting which is where most of the money is. Because Australia has a more strict implementation of an IR35-a-like law, contracting isn't as popular, which drives up perm salaries? That's my theory, anyway.
Plus, the Aussie dollar is still overvalued... I hope. I sort of expect it to eventually correct to somewhere between 2 and 2.5, especially if we stay in the EU.
context: I'm an Australian who moved to the UK in 2007, possibly moving back to Australia late this year or early next year.