There are a number of spots around Rotorua's Lakes were you can steal a little skinny-dip. Some are warm spots; thus generally, the most popular places.
This wonderful spot is quite hard to find - you have to know where to look for the little 'hole in the bush' which is the entrance to the short (but rough) track to the waterfall. It's quite near the last corner on the paved road to the Lady Knox Geyser. The waterfall is about 2 km from the more popular spot shown next. The hot waterfall is only metres from the Lady Knox Geyser
Since these hot pools are 'natural' they don't have temperature controls! Beware of straying from the 'used area' - if you do end up having to pull your foot out of boiling mud, you may have some trouble locating cold water in a hurry.
Between Rotorua and Taupo, in the middle of the North Island, are several hot pools and streams, which are not (yet!) commercialised. This means that if you're there first you can set the dress code. (If textiles are already there, it may pay to ask.)
In the middle of the North Island is Lake Taupo, and only about 5 km south from the town centre, you can approach this 'bay' from either end; and walk 100 m to the spot shown here.