Very different culture. Much more free and easy, so if things not being done "properly" is going to wind you up, don't go.. The australian government created a little character called "Norm" and the ad campaign was "sometimes near enough isn't good enough"
Work out the things that are going to make you miserable and take steps to mitigate.. invest in airconditioning, or a fabulous view if you need to remind yourself why you came.
Stick to the edges of the country if you want a sea breeze.
The tap water is lovely. What's limescale?
Work out what sort of feel you want the city to have. Brisbane is much more shorts and T shirt than Sydney, which is more cospopolitan and laid back than melbourne, which is more vibrant but more straight laced (more British if you like - people will queue in melbourne. They won't in sydney). Adelaide is more of a backwater, and Perth is like Sydney was 40 years ago. Darwin is bloody hot and full of men.
The sense of humour is very different, very dry and very pervasive. People let things go more and care less than the british about most things.
Don't underestmate how bewildering it is when you know what you want at the supermarket but have no idea what it's called.
Be prepared to travel.. everywhere is a long way away, except that isn't far in australian terms. You can drive for 2 hours in sydney, and still be in sydney.
Always keep enough in your bank account so if it all gets too much you can get on a plane home. Just knowing it's there is probably enough to stop you needing to use it.
Go with the right frame of mind, and you'll have a fantastic time. Underestimate how different every single thing is, and you'll dissolve in a pool of frustration.