In NSW, you will need an engineer's certificate to certify that it meets the ADR's; this involves fitting child anchorage point(s); side intrusion bars in the doors; non-convex mirrors; 'unleaded fuel only' sticker on fuel flap; and retractable rear seatbelts. Some engineers don't bother with the mirrors or fuel sticker...just depends on who you go to. There might be some other minor things, but the above is what I had to do for mine. Note, that this is separate from any non-standard modifications which will also need to be certified in order too get a roadworthy (blueslip).
Once you have the engineers certificate, get the car blue slipped, and get a greenslip for it. Take the engineers certificate, the green slip, blue slip and the car's receipt/proof of purchase as well as the import approval to the RTA and get the car registered.
Then, enjoy!