A mass-stranding of whales on a New Zealand beach has left 36 of the creatures dead and threatens 40 more.

Department of Conservation area manager John Mason said 99 pilot whales stranded themselves on Farewell Spit on the South Island.

By today, 36 whales had died and another 40 remained stranded and were still in danger.

Mr Mason said conservation staff and volunteers had successfully refloated 17 whales, which had swum out to deeper water. Another six whales remained unaccounted for.

The 40 beached whales were briefly swimming in shallow water early this afternoon but became stranded again by the evening as the tide went out.

Mr Mason said volunteers would try to keep the whales cool and wet until dark. He said after that, all they could hope for was that the whales would swim away on the next high tide during the night.

Pilot whales grow to about 20 feet and regularly strand themselves in large numbers during the New Zealand summer months.

Experts describe Farewell Spit as a whale trap due to the way its shallow waters seem to confuse whales and diminish their ability to navigate.