It's a smorgasboard of spots, really!
And foiling opens even more of them.
Need a bit of local knowledge, so get in touch with the fakeboobs local groups to tap into that. Jan/Feb isn't cold and a 3/2 wetsuit will be more than adequate pretty much anywhere. Sea temp typically around 18-20C that time of the year.
Just ignore all the East coast south of Christchurch to Oamaru. Looks awesome on the maps, but typically a very big gravelly dumpy disappointment at the end of the dusty roads. Waste of time when there are true jewels south of that.