WELLINGTON, Dec. 6 -- All 39 pilot whales stranded on New Zealand South Island's Farewell Spit were dead Monday, local press reported.

They were first spotted in shallow water off Abel Tasman National Park on Sunday afternoon, but the Department of Conservation said high winds prevented boats from being sent out to herd them out of danger.

When DoC personnel returned to the spit Monday morning, all the whales had stranded and 12 had already died, Radio New Zealand reported.

The remaining whales were then killed as they were too high up on the beach and the tides were dropping.

DoC said on Monday the stranding of whales on Farewell Spit is not uncommon in summer. AAP reports whales often beach themselves again after being refloated.

Pilot whales are regular mass stranders on New Zealand shores, especially in the peak stranding season from November through to March.

Strandings are common on the 24-km-long Farewell Spit at the top of the South Island.