It depends what the defect is for as to weather you get a fine or not. Wheels to large and too low e.t.c. do not incure a fine however if your exhaust is too loud or you have a pod filter then they can fine you up to $5000 for "driving a vehicle creating excess noise" or "modifing your induction system" but it's usually about $200 each.
I took this to court and after 3 court appearences representing myself and a lot of friggin around I won!
man you should have seen the copper's face, he got the shits and stormed out of the court room.
According to the australian road transport regulations act a defect should only be issued if something on you car would constitute an "imminent and serious safety risk to a person, property or the enviornment" otherwise you should be given a formal written warning which to my knowledge doesn't require you to get it cleared.
So things like "wiper blades contact sticker on the windscreen" should absoloutly not ever get defected as in my case.
Here are some usefull documents:
for wheels: Vehicle standards information. Sheet No 9. (RTA web site)
turning diameter, T.V's,lights, wipers e.t.c. :Road Transport Regulations 1998.
Exhaust and induction emissions (gas and noise): Protection of the enviornment operations regulation 2000.
Hope this helps. I have heaps more info on this stuff, should I start a special thread for it?