Hi Marc
Vehicle registration can be cancelled voluntarily at anytime by the registered operator and a refund claimed on the unused portion of the registration. The number plates must be either returned to any VicRoads Customer Service Centre or placed in self retention.

Here is the refund form.

As for selling a car in WA
Complete the vehicle licence transfer form with the seller.You will need to complete a Notification of change of ownership (MR9) form with the seller as you both need to sign the form.
The seller is required to submit the 'seller's copy' (green) to us within 7 days and give the 'Purchaser's copy' (pink) to you.

Important note:

If you choose to download a copy of the vehicle transfer form it will not be carbonated. The 'sellers copy' and the 'purchasers copy’ must both be completed and signed by you and the seller.


Notification of change of vehicle ownership (Form MR9)



I was able get the above information from two of the road authorities for each of the state.(VIC and WA)

Victoria
www.vicroads.vic.gov.au/ 13 11 71

Western Australia
www.dpi.wa.gov.au/ 13 11 56