the engine capacity from an A12 to A15 goes up 300cc. In NSW you would have to obtain a certificate for any rise in engine capacity equal to or greater than 200cc. With this change a upgrade of brakes suspension is usually looked at as well to cope with the projected extra power you are attaining from the cc increase. As it is it would be approx 15-20% rise so that would have to be reflected in a 15-20% rise in braking capacity, strength in struts etc etc...
Its up to you and who you get to do you roadworthy certificate each year whether this would be simply be overlooked .....or some of the inspectors like here in NSW would give you a hard time over the increase.