As KiwiFlyer said, Harbourside is very good for seafood. For somewhere a little more casual try the Auckland Fish Market (far side of the viaduct).

For Christmas make sure you book early otherwise places will book out. For a traditional Christmas meal try somewhere like Partingtons (Langham Hotel). I've had Christmas there a couple of times and always very happy - thinking of doing the same this year as will be in AKL.