Have you looked at BbTrumpet's excellent thread "OneUp results summary thus far"? Interestingly that would seem to indicate that the average successful Y>PE bid is $300 (while the average unsuccessful one is $462, but with a much wider range between minimum and maximum bids).

That would seem to indicate that:
1) Your bid is definitely high enough to be well in contention
2) If the seats just aren't available, then your bid won't succeed no matter how high it is.

I would have thought $500 is well above average for what people are likely to bid, despite what AirNZ's meter says (which, after all, is just designed to make you pay more) so I'd stick to your guns and hope for the best.

BTW, did you bid $500 USD or NZD? All the figures above are in NZD, so if you bid $500USD then that's more like $600NZD, ie a very strong bid.