The system is efficient.
The exhaust system does not pass closer than 25mm to fuel tanks or the body floor.
The system does not foul the body work or limit axle travel.
No part of the system projects below the rim lower edge.
The system terminates behind the last body panel.
Systems on the left hand side of the vehicle do not deflect gases directly to the left.
The noise level of the modified system must not exceed the noise level of the manufacturer’s original system by a noticeable amount.
Side exhaust systems (sometimes called lake pipes) are not permitted.
The system must be effectively supported and must be free from cracks and leaks.
Where the vehicle is subject to ADR 27A - Emission Control. Extractors may be fitted, but the features of the original manifold must be maintained (i.e.. hot air transfer tube, manifold to air connection). Vehicles subject to unleaded fuel control emissions (ADR 37) must remain as specified by vehicle manufacturer..
Noise level will be the most important factor in assessing any exhaust system. Close attention will be given to this factor during examinations and in the on-road situation."
Looking for the ADR link to back it up, as this info is a year or so old now.
EDIT: Found the new ADR's, and that rule has changed. You were right, it is now 100mm. ADR43
http://www.infrastructure.gov.au/roa...s_3Feb2006.pdf