The EPA website has a list of approved testers
http://www.epa.vic.gov.au/Noise/testers.asp
I take it the cop just defected your mate for the loud exhaust - if the car is under the limit, any fines issued for alleged non compliance would be void, right? It makes sense that the car owner gets the opportunity to remedy the alleged breach - and if they don't, then they get fined. When I got mine tested about 5 or so years ago, the test was about $60, and the place I went to in Box Hill - Daalder exhausts - didn't charge for the re-test as long as I got remedial work done by them - because it failed the initial test.
The policeman's testing environment and accuracy could have holes shot right through it if the matter was pursued further. An independent test would be the first step.
One lesson I learned when I got my mild sports exhaust on my XE was that next time I get a non standard exhaust, I will make sure they guarantee it is under the limit, so if it isn't, I can get is remedied at the vendor's expense! If an exhaust shop can't guarantee a sports system is legal, then they can't be any good and it's best to go elsewhere.