But scientists from the University of NSW believe they have located a massive, previously undiscovered group of the ocean giants thanks to bomb detectors.

It was the whales' powerful singing - recorded by underwater microphones designed to pick up nuclear tests - that gave them away.

"Humpback whales are like jazz singers," says marine ecologist Professor Tracey Rogers.

"They change their songs all the time.

"Blue whales, on the other hand, are more traditional. They sing very structured, simple songs."

The researchers suspect the population of pygmy blue whales - which still grow up to two bus lengths - is based in the central Indian Ocean.

But their unique song - which is so strong it can travel up to 500 kilometres - was picked up on recordings as far north as Sri Lanka and as far east as the Kimberley in Western Australia.

If visual sightings confirm the Chagos population, named after an archipelago near where they were found, they would become the fifth population of pygmy blue whales to be discovered in the Indian Ocean.

With blue whale populations pushed to the brink by whaling, that is a big deal, Professor Rogers says.

Unlike many whale populations in the southern hemisphere, blue whales haven't recovered from whaling.

Discovering this new population is the first step to protecting it, the researchers say, and opens the door for more research into the shy animals' behaviour.

"Blue whales in the Southern Hemisphere are difficult to study because they live offshore and don't jump around - they're not show-ponies like the humpback whales," Prof Rogers said.

"Without these audio recordings, we'd have no idea there was this huge population of blue whales out in the middle of the equatorial Indian Ocean."

Australian Associated Press