The world's rarest whale is believed to have washed ashore on a New Zealand beach, the country's conservation authority said.

The whale is so rare that since the 1800s, only six specimens have ever been known to science, the Department of Conservation (DOC) said.

The 5-meter-long (16.4-foot-long) whale, found on July 4 on a beach near Dunedin in the South Island, was believed to be a spade-toothed whale, DOC Coastal Otago operations manager Gabe Davies said.

"Spade-toothed whales are one of the most poorly known large mammalian species of modern times," Davies said.

"From a scientific and conservation point of view, this is huge."

A specimen this fresh offered the first opportunity ever for a spade-toothed whale to be dissected, Davies said.

DOC was working with local Maori, Te Runanga o Otakou, about what to do with the whale as it was a "conversation of international importance," DOC said.

"It is important to ensure appropriate respect for this 'taoka' (treasure) is shown through the shared journey of learning, applying mātauraka Maori (Maori knowledge) as we discover more about this rare species," Te Runanga o Otakou chair Nadia Wesley-Smith said.

Genetic samples had been sent for analysis, but it may take several weeks or months for the DNA to be processed and a final species to be confirmed.

The whale is in cold storage to preserve the remains until the next steps have been decided, DOC said.