The ozone layer sits about 9-19 miles above the earth, filtering harmful ultraviolet rays that can cause skin cancer.

Industrial chemicals containing chlorine and bromine used in refrigerators and aerosols have been blamed for thinning the layer because they attack the ozone molecules, causing them to break apart.

NIWA scientist Stephen Wood cautioned against reading too much into the hole's smaller size, which he said was also influenced by natural variations.

"We need to see smaller or less severe ozone holes over a number of years before we can say for certain that ozone is recovering," he said in a statement.

The only inhabited area that might possibly be affected by the hole would be the southern tip of South America, he said.

Under the 1987 Montreal Protocol, more than 180 signatory states have committed to phasing out the use of nearly 100 ozone-damaging substances.

In 2002, the ozone hole suddenly shrank, raising hopes it had turned the corner and was starting to close but some scientists later put it down to an abnormality caused by atmospheric conditions.