Last year the hole reached a record 30 million square km (11.6 million square miles) - three times the size of the United States.

This year saw a slightly smaller 26 million square km (10 million square mile) hole at its peak in September but it was more stable and likely to last longer, the National Institute of Water and Atmospheric Research (NIWA) said in a statement.

"The longer the ozone hole persists, the more likely it is that ozone-depleted air from the dispersing hole will reach New Zealand when the sun is high in the sky, increasing the risk of occurence of periods with high ultraviolet radiation," NIWA said.

The NZ government institute warning was aimed at New Zealanders but NIWA scientist Stephen Wood said from the NZ research station at Scott Base, on Antarctica's Ross Ice Shelf, that the issue applied to other areas of the temperate southern hemisphere band, in latitudes from around 30 to 50 degrees south.

"When it breaks up you get little filaments, or remnants of the ozone-depleted air that go across the mid-latitudes," he told Reuters.

OZONE LEVELS DOWN

Ozone levels over the southern hemisphere's temperate zone have dropped 15 percent over the past 20 years, Wood added.

"The dilution effect from the ozone hole in the Antarctic is responsible for around half of the changes in UV that we've seen in the southern hemisphere," he said.

Ozone molecules, made up of three atoms of oxygen, form in a thin layer of the atmosphere around 10-29 km (6-18 miles) above the earth and absorb ultraviolet radiation from the sun.

The ozone holes are triggered by a combination of chlorine pollutants in the atmosphere, extremely cold temperatures and the return of sunlight to Antarctica in spring.

A reduction in chlorine pollution has scientists hoping that current ozone holes are at their most severe, but they say it could be 50 years before levels are restored to normal.

"Our research has suggested that climate change could further delay the recovery in ozone levels," Wood said.

This year's hole has a minimum ozone reading of 132 Dobson Units over Scott Base - compared with last year's 126 units minimum and a typical 300 units in other parts of the world.