Australia introduced The Australian Design Rules (ADRs) in Jan 1969 . These are national standards for vehicle safety, anti-theft and emissions. The ADRs are generally performance based and cover issues such as occupant protection, structures, lighting, noise, engine exhaust emissions, braking and a range of miscellaneous items. ALL vehicles used on public roads in Australia MUST comply with those rules as they applied on the date of vehicle manufacture. Magic! My Vincent was made in 1951 so no part of the ADR's applies (as they did not exist at the time) - this is especially good in regard to engine exhausts. As long as my bike is fitted with a silencer device then its legal. It does NOT matter how effective or ineffective it is - it just needs to be there.
You do not need to be a member of Mensa to figure out that a plain, straight through, totally non-restrictive exhaust can be totally legal provided the vehicle is made BEFORE 1969 and it "seems" to have a silencer in place.