Just in case you're wondering as I very rarely send snail mail so have stamps from 2006...

It's 50c to send standard envelopes via standard NZ post mail and I was curious if they refuse to deliver if it's only got a 45c stamp on it. I rang customer support and they can and sometimes do refuse to send short mail! It's just at the liberty of the mail sorter and they do sell 5c postage stamps to make this up.

So there you go, add that to your useless info pile.