From RTA Web site. Minor changes to wheels Wheels up to 26mm wider than the largest optional wheel recommended by the vehicle manufacturer for the vehicle can be fitted without the need to notify the RTA. The outside diameter of the wheel and tyre combination must be no more than 15mm over the largest diameter wheel and tyre combination specified for the vehicle and not more than 15mm below the smallest diameter wheel and tyre combination specified for the vehicle. Replacement wheels requiring an engineering certificate Wheels that exceed the diameter and width limits for minor wheel changes must be assessed by a signatory. If the signatory finds that the wheels are safe for your vehicle, you will be issued with an engineering certificate. There are limits on the size of wheels that can be certified by a signatory. See below, http://www.rta.nsw.gov.au/registration/dow.../vsi09_rev4.pdf This rule applies to trucks and busses,..........cars? Ground Clearance A vehicle or combination must have a ground clearance of: a. at least 100mm at any point within 1 metre of an axle; and b. at least one-thirtieth of the distance between the centres of adjacent axles at the mid-point between them; and c. at any other point - at least the distance that allows the vehicle or combination to pass over a peak in a road, the gradient on either side of which is 1:15, when the wheels of one axle of the vehicle or combination are on the slope on one side of the peak and the wheels of the next axle are on the slope on the other side. I was led to believe it was 100mm between the metal bodywork and the road. There was/is also a minimum headlight height. 520mm spirngs to mind but dont quote me.