RTA specs are 1 inch taller and wider than largest stock rim available on that model of car (ie how big were the GTI mags) is legal without an engineers certificate
2 inch's taller and wider then the largest stock rim available on that model of car a with one. 18 inch rims on the rolla is a waste of money to get the same rolling diameter as stock the tyres will be a very low profile have a harsh ride and not have a great relationship with australian roads, pot holes etc.
Not to mention the car will look like a roller skate.