Heavy Vehicle (Vehicle Standards) National Regulation (NSW)

A heavy motor vehicle propelled by an internal combustion engine must be fitted with a silencing device through which all the exhaust from the engine passes.

For the purposes of subsection (1), a silencing device designed to be manipulated by the vehicle’s operator, including, for example, by in-vehicle controls, must be designed so that it can be tested in its worst case configuration.

Note for subsection (2)—

A device is tested in its worst case configuration if it is tested in a way that produces the loudest noise. For example, if a butterfly valve is used as a silencing device, it must be able to be tested fully opened.