Thirteen whales have beached near Farewell Spit, near Golden Bay.
A pod of around 50 whales were spotted this morning, when some became stranded around 7km from the spit, said Department of Conservation (DOC) spokesperson Greg Napp.
Eight DOC staff are with the whales at the moment, with some Project Jonah volunteers on the way, he said.
"High tide was at 9.30 this morning, so the next high tide is 9.30 this evening - which means we have a short space of time to get them floating again," he said.
"It's a long time for them to be out in the sun."
He said they would need to be back in the water between 8.00pm and 8.30pm, before sunset.
The rest of the pod are offshore, but are still not out of danger yet, Napp said.
"We have a boat in the water to intervene if we need to, because they could beach at the low tide mark," he said.
DOC would use the boat to direct the other whales from danger.
Have you adjusted to the new alcohol limits for drivers?Related story: New alcohol limits catch first drivers