IMHO the chance of a true op up, on any NZ flight, on any given day are less than 1/10 of 1%. Slim to none. If you're wanting to be upgraded - then I think the safest choice is a One Up, and take a look at the latest results to give you an idea of what's worked in the past for others. A low One Up bid certainly against the averages that are posted is probably as good as not bidding at all. Light loads and a low bid don't necessarily equal it being accepted - I've tried and failed at that strategy.
On another note - it is a good idea for a thread of its own - NZ op-up history. I would be intrigued if op-ups happen with NZ very often at all!
I've been fortunate to have some on EK, QF (way back when there was no JQ), and once on NZ in about 2002. I just don't think it's in NZs manual now! :-(