About 20 km northwest of Wanganui, Ototoka is reached via a fenced, gravel track through a farmer's paddock. The nude area is a 3-400 m walk north west (though some prefer the southeast!) around the point visible from the car park exit point (at the top of the cliff).
South of Wanganui are several other possibilities for skinny-dipping, the first being South Beach, next to the airport.
Foxton has a stable car access to the beach, so you can drive onto the sand and go 10 km north from there to the middle of Himatangi Beach which also has stable access. On the way, you'll notice lots of driftwood, just inviting construction of a DIY shelter!