New Zealand officials say at least 22 pilot whales have died after being beached on the country's South Island.

Rescuers and volunteers on Farewell Spit have been trying to keep the remaining 77 whales alive, keeping them covered and watered.

Officials say if they do not return to the sea at high tide on Monday night, there might be another opportunity to refloat them in the morning.

Department of Conservation spokesman John Mason says the teams will be back at the beach first thing in the morning.

"The hope is they'll refloat themselves at high tide tonight," he said.

"Given our past experience, we're not overly optimistic that this is going to happen, so we'll be out there tomorrow morning at first light, and if they're still there, we'll work towards having an assisted refloat with our volunteers."