Close to 50 long-finned pilot whales are stranded on a remote beach at the top of the South Island.
A Department of Conservation spokesperson said so far nine of the whales have died.
Two-thirds of the whales are alive, Project Jonah said on Facebook.
DOC was first alerted to the mass stranding at the base of Farewell Spit in Golden Bay by a tour operator at 9.30am today.
The department said rangers are on site and they're aiming to refloat the whales once the water is deep enough.
Around 50 Project Jonah marine mammal medics are either caring for them on the beach, keeping them cool and wet until they can be refloated or are on their way to the site to help.
The last mass pilot whale stranding on Farewell Spit was in February 2017 when up to 700 whales were beached.
About 250 died while the rest were refloated.
Project Jonah said beaches are also being checked for other whales that may have stranded further along the coast.
No further help was needed at this time, a spokesperson said.