We ended up going for a Christmas dinner at Halia, and while (as expected) it wasn't very Christmasy, it was an excellent restaurant in an amazing location. The verandah outside, within a view of the floodlit jungle all around, was especially nice. It wasn't cheap either -- count on $100/head for a full meal with drinks -- but definitely worth it for a special occasion.
The hard part is getting there, we had to drive around and make a couple of calls before we found our way. The entrance is on the 'wrong' side of the gardens, so you have to park/get dropped off at the Tyersall Ave entrance (sign says "Ginger Garden"), and then walk a short distance into the Gardens to find it; the first signs/map for the restaurant are only visible on foot.