I've been to Australia a few times (once with JY), and it's such a vast country it's hard to be exact about temperatures.
November/December is late spring/early summer, and is probably a good a time as any, especially as you "escape" some of the winter in the UK.
Having said that it can reach the mid 30's (and higher) in many parts of Australia anytime from November to February. For example, I was in Perth & Adelaide January this year and most of the time it was into the low 40's.
I love Australia, and I'm sure you will.
Stuart