Most restauratns close early on Christmas Day so staff can have some time with their families (so the few open for dinner may close 7pm for example and most only offer lunch even if it is called dinner). Imagine there would be at least 1 delivery pizza left, and the larger hotels will have a kitchen open ... but it isn't like a big city. You may want to investigate when you arrive in ZQN, chances are your Milford Sound tour returns late after most things are closed.