The exhaust thing (plus other ADR items) isn't that hard to work out. If one wishes to replace the OEM exhaust with an aftermarket one then it must meet the ADR requirement ( or other standard) in force at time of vehicle manufacture. If the OEM exhaust has a cat convertor (under ADRs) then the aftermarket one will need one as well to remain ADR compliant otherwise it can be defected.
Noise
The amount of noise a motorcycle can produce from the exhaust outlet depends on the year it was manufactured. If it was manufactured after February 1985, the acceptable noise level is 94 dB(A), but if manufactured prior to this date, then it is 100 dB(A).
The noise level is tested with the engine operating at a steady speed and, depending on the type of engine, the engine speed will be between 2500 rpm and 3750 rpm
South Australian viewpoint
http://www.sa.gov.au/topics/transport-travel-and-motoring/motoring/motorcycling/motorcycles/motorcycles-and-sidecar-requirements#title6If modifications are being considered you must ensure that the vehicle's ADR compliance is not compromised by the intended modification.
Motorcycles manufactured after July 1975 are built to comply Australian Design Rules (ADRs). These design rules cover various aspects of the motorcycle such as brakes, noise and lighting.
Motorcycles that comply with the ADRs, and were manufactured after October 1977, are also fitted with a compliance plate. This plate is mounted to the motorcycle's frame, usually in the head stem area.
If a motorcycle is certified to comply with the ADRs it must continue to comply with those rules. If modifications are being considered you must ensure that the vehicle’s ADR compliance is not compromised by the intended modification.