Thirty-nine pilot whales from a pod of 99 that stranded on Monday have re-stranded off Farewell Spit in Golden Bay.
Whale rescue group Project Jonah says 26 of the original pod refloated themselves on Monday night's high tide and appear to have left the bay, while 34 others have died.
The group's chief executive, Kimberly Muncaster, says volunteers are keeping the whales wet on Tuesday and preparing them for high tide about midnight.
Ms Muncaster says pilot whales are robust as a species and cope quite well with strandings.