I think I would head down once over the new year period.
The thing is during that time of year, there are so many tourist floating around that the Marlborough sounds is a disaster zone and any trout river would be busier then many shopping malls.
So the only form of fishing left open is to head down to the surf beaches. Sure there would be more people with the same idea but the long beaches offer plenty of space.
So I would be concentrating on my trout and snapper fishing for the first half of December, with maybe a single trip down to kaikouria.