Chainrings are easily and often replaced. The presence of the chainstay protector does add some additional credibility but I still don't have a high degree of confidence.

I wouldn't make much of a case on it being a 1984 based on the SunTour derailleur codes, as they would appear to be replacements. The Columbus tubed Nishiki models were an attempt to crack the high end market after they failed to do so in the 1970s. First they tried with high end Tange frames equipped with Campagnolo Nuovo Record groups. When that didn't work well they moved to Columbus frames equipped with Nuovo Record groups. In this respect, it doesn't make sense to see a 2nd tier Japanese group like Cylone. Even if the Japanese components were a concession to the Austalian market, I would have expected to see top tier Superbe Pro (like the brakes) or Dura-Ace. Based on the mixed Japanese components, it's a Frankenbike to at least some degree and the derailleurs are almost certainly not OEM.

Assuming it is a Nishiki, the two known Columbus tubed models were the Maxima and Cervino. The former was made in Japan and the latter made in Italy, so you can tell the difference via the bottom bracket threading which should be marked on the cups.

The Australian market certainly poses extra questions, given that few Australian market models have surfaced on the forum. It may be a Nishiki but unfortunately I can't corroborate it, based on the current evidence.

BTW, please accept a slightly belated welcome to the forums.