The family and I did a Xmas cruise 3 years ago. Flew to Auckland on the 19th. Poked around there then boarded on the 23rd. Bounced down the east coast, Milford Sound then Hobart for NYE before a day ashore in Melbourne (we just strolled down to Luna Park and St Kilda shops because we have been a few times). Finishes Sydney so one way flights only.
Was a great minimilist Xmas. Small presents for the kids. Lunch with no cleanup. No other relos. I enjoyed it as a once off.
Ive done two cruises. I dont mind them as long as they are 4-5 years apart. I am happy running laps of the ship and hitting the weights room, plus quite happy to get messy on the drink every now and then so it is ok for me. If you are a health freak it wont be your thing. Food is like eating in a shopping centre food court for a week or so.