Are you on the Kapiti Coast? If so your channels have just been restacked.
A rescan should find the new frequency, assuming there isn't something interfering on that frequency eg an old VCR RF output if you have it looped through. 70% signal is a bit low actually. It's quite possible that the TVNZ signal is too weak, and the lower frequency it is now broadcast on is not as optimal for your antenna / signal path.
Do you have a decent UHF antenna?