Earlier this year we spend a month cruising to New Zealand and back up to Cairns.

New Zealand is fantastic, particularly the South Island. With your limited time you may be able to visit Fjordland (Milford Sound etc) and Dunedin, which is on the Otago peninsula. Apart from Sydney, which I love, Dunedin is a fantastic place to visit. Also on the South Island, Christchurch is well worth a trip. As for the North Island, it was OK but Auckland nor Wellington were anything special.