Even if it was parked next door to the power plant Terry, any fallout would've turned to regular old dust even before it was washed for transport to Aus. Quarantine has been quietly doing checks on cars coming in from Japan recent times and has found nothing.
Water damaged cars aren't a problem if they're completely stripped down and rebuilt, but the people who tend to buy them don't buy them as projects, they buy them to make a quick buck. As a result, they take short cuts and you end up with cars that work fine when you buy them, then smell mouldy and start having random electrical problems down the track.
Water damage generally comes from rivers, and being fresh water it doesn't cause an issue for rust. In the case of the tsunami, however, there are plenty of near-the-ocean cars that were borderline rusty that will now be much worse as a result of getting salt water through them. The flooded cars were already at auction under a week after it happened, but they're clearly marked and most of them are Kei cars and normal Japanese runabouts, not top end sports cars.