Check out page 10:

http://epanote2.epa.vic.gov.au/EPA/P...;FILE/1031.pdf

It is an offence to own or use a vehicle fitted with a temporary defeat device, inlet port restrictor,
exhaust port restrictor or temporary noise reduction device. These devices, when fitted to the exhaust
system, can temporarily and easily change or vary the level of noise emitted from the exhaust.
Examples include variable exhaust restrictors, cutouts, bypasses, adjustable/variable/sliding
openings on mufflers, restrictive or temporary insert plates and steel wool.