Happy to be corrected here...
There is nothing in law preventing it. But I know of no airline that allows anyone other than persons who work for that airline and hold a valid ASIC to enter the cockpit. Gone are the good ol' days. Most will let you in on the ground, so if you're easily entertained, try that.
If you happen to be looking for a little cash or part time job, look at National Jet (I see you're in ADL). Few months with them behind a desk and you should be able to get on a freighter (with an ASIC) to MEL, BNE or SYD. Unless things have changed there in the last 6 months...