Central Whirinaki is a large 25 bunk hut, sleeping 11 in each of two bunkrooms, plus 3 in the large kitchen / dining area. The hut is 'serviced' category, but no firewood or gas is provided, so it's unclear what the service referred to is - maybe the presence of a warden in summer?
Large flats outside provide space for camping. there's a woodburner - wood may be an issue - the area had been stripped pretty bare when I visited. Tank water is plumbed, and the river is 20m away if you prefer a swim.
A very popular hut - I recommend a rainy midweek day in winter if you want it to yourself!