Not illegal in Victoria
Section 13.2 VSB 14 lists that you can substitute suspension components without certification - provided amongst other things it is not raised or lowered by more than 1/3 of its initial suspension travel. No mention of coilovers as exclusions.
Cars with ESC may be different - as lowering or raising may affect the way the
ECU responds to its set programming ? This is more to do with parameter alterations than hardware change - so it would apply to lowering / raising the vehicle regardless of coilovers or using lowering springs.
This doesnt stop the boys in BLUE questioning anything they want to question - just means you are obliged to prove what you have modified is within the rules legislated by your state.