You will love it! My wife and I went from Auckland to Sydney last January on the Star Princess. We stayed 3 days pre cruise in Auckland and 4 in Sydney. Sydney and Auckland were both friendly and wonderful cities to tour.
January is peak season and the weather was good throughout the cruise with the exception of rain in Fjordland and a few rainy periods elsewhere in NZ. We never needed more than a light jacket. As you go farther north, the weather gets warmer as you are getting nearer to the equator. If you are planning a trip to the Great Barrier Reef, I would not plan to go in late January or Februare as it is oppressively hot and humid and very crowded during the peak season. We loved Sydney and plan to return and continue exploring Australia. We can hardly wait to go back!
I have a pretty good file of information that I would be happy to share with you. It could be useful.