Well, my 32 has a mod plate in it from 2005. So, after yours, but only one plate. The blue one - with an LO1 mod code on it.
I know NOW that they have purple colored ones, but I don't know when that came in exactly.
Obviously QLD rego rules are different from SA ones though, as it was perfectly fine for my car to be registered.
I wouldn't say that it was "one of the first brought into Australia" though if it was complied in 2004. Skylines have been here a LOT longer than 6 years. There would still be quite a lot of 89 models over there (mine got done in 2005 for example, so it came over after yours) and I'm sure some R34s would have beaten both of ours over.
Looks like SA Transport just have stricter rules, so it's possible that even with a purple plate they might have still found something to bitch about.