I'm not an importer, but the rule has always been the same: As long as you don't import a car manufactured WHILE it was locally available here in Australia, then you're fine.
Example 1: the R32 GT-R was available from 1989 through to 1993, but was only officially sold here through Nissan in around 1992, so anything before or after the time they were OFFICIALLY on sale via Nissan Australia is okay to bring into the country. You can also bring in any(?) GTS or GTSt R32s because they were never available here at all...
Example 2: the Z32 300ZX was never officially brought into Australia with the twin-turbo VG30, so you CAN import one of them because it's different from what Nissan offered.
Example 3: the S15 Silvia was also only brought into the country for a certain period, so anything outside that time is fine. Also, the S15 Varietta was never available at all, so you can get one of those too (but they are pretty rare, and are all non-turboed SR20s with an auto-box).