The whale watch tour claims to have a 95% rate of viewing a spem whale at any time of the year. They operate days and offer a 80% refurnd of your tour costs if you do not see a whale. You can check out there website and rates here
http://www.whalewatch.co.nz/. I've taken one of there tours before and we saw quite a few sperm whales and one other type of whale, just can't recall its name at the moment.