Whilst I am from Canberra, I have lived in Sydney and can recommend a harbour cruise as one of the best ways to see the major parts of the city. You will get to see the major attractions - then just having a meal and watching the action on the harbour at Darling Harbour is not a bad place to start.
My wife and I are on the cruise leaving Sydney on 11 January and are counting down the days. Our last cruise out of Singapore was a little eventfull, with the Pacific Sky breaking down - the Sapphire has a much better reputation.
We have an extra night in Auckland at the end of the cruise - so if anyone has a hotel recommendation in Auckland, please let me know.