Depends to a certain degree on the cruise line and the carriers although most use schedule airlines. Talk to your travel agent because it is easy to arrange stop-overs. They will probably help you with a hotel but transfers are normally up to you. We love Singapore and a taxi to the hotel there is very reasonable. Just ask....Id.
What I would recommend is speaking to your travel agent and asking them to tailor make a package for you. They will put flights, cruises, transfers together however are best for you, and yes you will be able to stopover on the way back. You will probably save money as well tailor making the package through an agent! The following cruise lines are the best to choose for cruising New Zealand/Australia: Celebrity, Princess and Holland America.