Assuming the cost is similar to the cost of shipping Australia to NZ,  you will be looking at a shipping bill of more than $3,000 (I considered shipping my car from Oz to NZ late last year).

Why would you bother, given new cars in Oz are so much cheaper new than NZ ?

However those issues are minor compared with the MAJOR problem you will have meeting Australian Design Rules requirements,  all cars in Oz are required before registration to have ADR compliance plate fitted certifying the vehicle meets the federal ADR's,  exemptions are available for cars built prior to 1970 (I think that's the cut off date).

Unless your car was originally manufactured, assembled or sold new in Australia and already has an Australian ADR Compliance Plate it may cost you thousands of dollars to get it modified & certified for ADR.

If you do decide to proceed,  be aware you need Import Approval from DOTARS (Aust Govt Dept) before the vehicle is shipped from NZ.

Depends on how long you have owned the car.  One of my previous bosses here  when he came from ozz to work  bought two 2nd hand import BMWs in NZ  and when he went back to ozz a couple of years later took them with him.  No duty charged but if he had bought them say  a couple of months before he moved back ...well the ozzy tax gathers would have been waiting on the dock  when they were unloaded..

check this link