a heal pump is least efficient on a cold night right around the due point in the 1-5deg range. so someone in Hamilton will have a less efficient heat pump than say someone in central otago where it is -10deg outside.
If your house is well insulated and you have double glazing it may be more efficient to leave it on all the time, than to turn it on and off.
Can you explain further?
On first reading I found what you were saying confusing mentioning dew point and actual temperature.
My first interpretation:
Hamiltion with higher humidity reaches dew point fairly early on (say +1C perhaps), whereas Otago with lower humidity does not reach dew point until about -10C.
I guess the dew point determines roughly when the outside heat pump coils freeze.
Am I on the right track?