We did Australia to New Zealand in January last year and we had great weather for most of the 14 day cruise, it was a bit wet in Sydney not long after we arrived but the other ports of call were fine. You will be there in the equivalent to our June/July and our trip was like our July/August, well when we get some good weather in those months. We found the New Zealnaders were much friendlier than their Aussie counterparts at least in Sydney, but not in the other ports of call in Australia. Most days my wife was out there adding to her tan so no problems there, possibly you might have warmer weather given the movement of the ship when at sea as you will be there in what is the height of their summer. We were concerned about the long flights but time went much quicker than we imagined and we had no travelling after effects going there or back but that might not be the case for others. Finally we had no rough days at sea during our trip but you can't forecast what December 2009 will bring but I would think it will be fine most days. We really enjoyed the trip and were glad we did it. We sailed on the Mercury a 4 star 1,900 passenger ship belonging to Celebrity.