Engineering certificates are still valuable... but you need to lodge them with the RTA to be valid or something. (Blueslip + engineering certifcate)
Not sure on who in Sydney... but we use Kevin Humphries up the coast (Wyong). The RTA website has a list of approved engineers.... one should be in your area. Be prepared to pass emissions though if you want to engineer a turbo back exhaust, aftermarket ecu etc.
Anything not on the engineers report can still be defected ! My mate has a sunny with a CA18DET in it. Car is completely engineered and the two times I have seen them go over it ..... it has taken them over an hour as they try and find things not on the certificates and also validate that everything listed is still identical.