WELLINGTON, Feb. 3 --- Scientists have said they are closer to understanding the world's biggest animal, also one of its rarest, after finding almost 50 blue whales off the west coast of New Zealand's North Island.

The whales, once hunted intensively in the Southern Hemisphere, were seen foraging for plankton over the last week in the South Taranaki Bight, the New Zealand government's National Institute of Water and Atmospheric Research (NIWA) said Monday.

NIWA marine ecologist Dr Leigh Torres, who is leading an international team of blue whale researchers that found the elusive animals, said they would start collecting important data on the population and the foraging habitat.

"In addition to finding the whales, we were able to detect their prey visually on the surface and at-depth using hydro- acoustics," Torres said in a statement.

The sightings of blue whales were linked to a prominent upwelling system that generated large clouds of plankton, on which the whales preyed, into the bight.

It was previously thought that the whales were only travelling through New Zealand waters while migrating, she said.

"Blue whales need to eat vast amounts of plankton to support their energy demands. But there are just four confirmed blue whale foraging grounds in the Southern Hemisphere outside of Antarctic waters," Torres said.

The research team had taken hundreds of photos and tissue samples to identify individual whales, acoustic recordings, prey samples and oceanographic data.

Fewer than 2,000 blue whales, which can weigh up to 150 tonnes, are believed to remain in the Southern Hemisphere, compared with an estimated 200,000 in 1900, according to the Department of Conservation.

The research team comprised scientists from NIWA, The Blue Whale Study based in Australia, and Oregon State University in the United States.