Call it a happy feat.

New Zealand scientists have been astounded to find that two species of subantarctuic penguin were able to travel 15,000km - equivalent to the distance between Auckland and Boston - over a stretch of just six months.

The insight was revealed by a tagging project observing nearly 100 subantarctic rockhopper and Snares penguins over winter in the Southern Ocean.

"If they are constantly moving this averages out at about 100km a day but you also have to add on to that the distances covered vertically as the birds dive to capture food," said the study's leader, National Institute of Water and Atmosphere seabird ecologist Dr David Thompson.

While the Snares penguin population on their craggy namesake islands was relatively stable, Campbell Island's rockhoppers had dwindled by at least 21 per cent since 1984, leaving just over 33,000 breeding pairs there.

The island was once the world's largest breeding colony of the colourful rockhoppers - the smallest of all penguin species and featured on the movies Happy Feet and Surf's Up - but between 1942 and 1984 the population dropped by about 94 per cent.