The hole in the ozone layer over Antarctica has increased to 27 million sq km, which scientist say is larger than it has ever been before.
The National Institute for Water and Atmospheric Research scientists had expected the hole to get smaller, because the atmosphere's levels of ozone-depleting chlorine are lower than usual.
However its current area is one million sq km more than its previous maximum size.
Dr Richard McKenzie of NIWA says the hole began developing late this year, but grew rapidly.
He says it will not affect levels of ultraviolet radiation in this country until later in summer, when the hole dissipates.
Dr McKenzie says the size of the hole has been fluctuating in recent years, and he does not expect it to increase next year.