How long have you got? The route
you're describing is nearly twice as long as the direct route
, by the time you've gone through Cook Strait and up the East Coast. So unless there's a cyclone hanging around up north, you're much better to go the standard passage
around North Cape. You can still swing down in a southerly arc
to catch the best winds if they're down there, and heading for New Plymouth is an option if there is a storm a-coming.
If you really want to sail the East Coast then you may as well clear at Wellington - no need to make the detour to Nelson unless you're sightseeing (or there's a nasty southerly you want to wait out).
As you say, weather
will be the final determinant.