Members of the public found some 30 pilot whales on Puponga Point beach on Friday afternoon. The rest of the pod then followed them, despite rescuers' efforts to stop them coming into shore.
Department of Conservation staff managed to float four of the whales back out to sea, but will have to wait for the next high tide before they can attempt to return any others.
Animal experts will monitor the whales overnight. The priority is to keep them well hydrated and protected from the sun.
Several volunteers, including some tourists, have joined conservationists in the rescue attempt.
Some of the whales will not survive the ordeal, said Conservation Department spokesperson Trish Grant. Even those that are successfully returned to sea may be so weakened that they'll die soon after.
There have been several instances of whales beaching themselves in the Golden Bay area, which is the northernmost point of South Island.
It's not known why whales become stranded. It could be that the shape of the bay, which has a long sand spit, confuses them and they get lost, says Grant.