Anything valued less than A$1000 FOB is duty free nad exempt GST, however second-hand gear is subject to quarantine. By mail it will generally be cleared at the mail exchange and then delivered, but by sea and airfreight it will require a SAC (self-assessed clearance) to be lodged. you can do it yourself, but it can be time consuming.
Cars and most parts are still 10% duty and 10% GST, which will phase down to 5% duty in 2010 (as long as there's no change of government). Some bits are subject to restrictions, such as tyres (should have a DOT mark) and seatbelts (ASA marks) but this is rarely enforced. Airconditioning is a real pain in the arse, and expensive to import due to licencing requirements by the federal dept of Environment and Heritage. I've been lobbying with other interested parties to change the import rules for some time, to no avail- the current system is more about keeping Ford, Holden, toyota and especially Mitsubishi afloat than any bollocks about safety and emissions.