First, the rice thing is a myth - you'll only end up with corrosion and water that evaporates at a rate that is no better than if you stuck it in your sock drawer. Note that all the guides say you should keep it in rice for a few days - you could also leave it on the table and achieve exactly the same result after a few days as well.

There are numerous guides and threads about drying wet electronics that don't involve rice, follow one of those, they are better. Drying should take no more than an hour or so, not days.

The N900 has a water ingress label - they'll just point to that if you question the legitimacy of their assessment.

Effectively, being in NZ, you'll need a new main board, this will cost you almost as much as the N900 itself unfortunately. If you're ever around Asia then drop in to one of the cell phone markets, someone will be able to repair it for you by swapping out only the dead bits.