I am moving to Aussie in a few months and have to decide what dive gear to sell off and what to keep. Does anyone know what thickness wetsuit the locals use. I have a 3mm "farmer john" style (2 piece) and a northern hemisphere compass that I’m mostly concerned with. I know tilt is a problem with compasses but am not sure how much it will affect me on the east coast of Australia vs further south. Will it read true but tilted, or be totally off.

There is not really any such thing as a "northern Hemisphere compass". A compass should work fine just about anywhere on the planet. All you have to do is check the local variation which will be different. You don’t say where you are moving to (north west Australia, etc) but if you’re thinking about that area, then your present wet suit should be fine. The only other thing I’d say is flog your tanks and weights before leaving. Very heavy to transport and readily available to purchase / replace when you arrive.