A conservation department worker noticed the whales washed ashore at the picturesque beach in Golden Bay on Thursday evening.
But the government agency decided against a night rescue effort for fear volunteers would be injured by the whales in the darkness.
Local media reported on Friday that volunteers had managed to refloat some of the whales during high tide, but most were quickly restranded as the tide ebbed.
The event was New Zealand's largest known whale stranding since 1985 when 450 were stranded in Auckland.
Whales often get stuck at Golden Bay, a remote but popular holiday area at the top of New Zealand's south island.
The bay's shallow waters make it difficult for whales to return to deeper water, according to marine life rescue organisation Jonah Watch.