Instead of following the usual pattern of hitting a maximum, then declining, the hole has stayed near its peak for weeks, even rising again last week, according to measurements made by NASA.

The US space agency's OMI satellite captured a maximum of 26 million square kilometres on September 12, but showed it rising again in recent days as ozone-depleted air repeatedly brushed the far south of South America. Larger than in the previous two years, it was still short of the record 2006 hole measured by NASA at 27 million square kilometres - the size of North America.

It was persisting under the influence of particularly strong prevailing winds in the upper atmosphere, the CSIRO's Paul Fraser said yesterday.

Dr Fraser confirmed trends still indicated recovery of Earth's ozone layer, while uneven, was continuing, with man-made ozone-depleting chemicals now about 15 per cent below their peak in the atmosphere.

''Year to year variability in the weather can effect the scale of the ozone hole significantly,'' Dr Fraser said.

The UN's Scientific Assessment of Ozone Depletion report concluded that the Antarctic hole drove changes in surface winds over southern hemisphere mid and high latitudes, and was linked to warming of the Southern Ocean.

It said as a result of the phase-out of ozone depleting substances, the ozone layer outside the polar regions should recover to pre-1980 levels some time before mid-century.

The spring time ozone hole over the Antarctic was expected to recover much later. An unprecedented large ozone hole also formed over the Arctic earlier this year, due to high levels of ozone depleting chemicals and unusually long cold spells, the United Nations' ozone secretariat said. It covered about 2 million square kilometres, according to scientists in the journal Nature, who said it was not possible to predict when such severe depletion could be matched or exceeded.