A very rare breed of whale has washed up on a South African beach, a marine scientist says.

Vic Cockcroft, of the country's Centre for Dolphin Studies, says the five-metre-long dead whale that appeared on a Western Cape beach last week is a Longman's beaked whale.

We know absolutely nothing about them

Vic Cockcroft, whale expert

"It's amazingly valuable, simply because we know absolutely nothing about the animals because they have only been seen two or three times alive," he said.

Only two other complete carcasses of this kind of whale (Mesoplodon pacificus) have previously been found, as well as three skulls. So the animals remain something of a mystery to researchers.

"We don't know the maximum size, we don't know where they feed or what they feed on. I mean we know absolutely nothing about them, where they occur even," Dr Cockcroft said.

The other two Longman's beaked whale carcasses found also turned up in South Africa, one a decade ago and the other in the early 1980s.

Three skulls have also been found in Somalia, Kenya and Tasmania.

Beak-like mouth

As its name implies, it has a long, beak-like mouth, and is believed to normally inhabit waters far from shore.

From the shape of their teeth, scientists believe the whales feed on squid.

Scientists are performing an autopsy on the dead whale, and samples of its flesh have been taken for genetic and other testing.

Its skeleton will reportedly be exhibited in a local museum.

It is the second odd creature to turn up on a beach in South Africa in recent months.

A rarely seen megamouth shark, a breed only discovered in 1976, washed up on a Western Cape beach just three months ago.