I have been looking for the exact legalities for wheels / tyres as well. Lots of people scaring with misinformation.
Try this document from the rms (2013) on what is a legal modification. Section on tyres is on page 7. Need to make sure your total diameter is not 7% greater or width not more than 25mm than stock.
http://www.rms.nsw.gov.au/documents/...ifications.pdf