If there's anything that interests you in each port, definitely book in advance. December is summer in NZ and if you don't book, you might miss out. In some ports there maybe selling of tours at the wharef - i.e. Tauranga, whereas in others you will need to make your way to the Visitor Centre. In Wellington, you might want to check out the special effects movie-making geniuses at Weta Workshop - pre-book the There & Back Again tour which includes transport from the Information Centre and the tour. And definitely pre-book .. it's one of the first to sell out on cruise day.