Unseasonably high temperatures are forecast to keep New Zealand warm for the last month of autumn.
The next week could see temperatures in the mid 20s and Invercargill is poised to break previous records.
"Invercargill might be [a record breaker," said Metservice spokesperson Emma Blade.
"But the whole country is going to feel very warm, particularly hot for this time of the year."
The warmer than average weather should last for at least the next three weeks.
Blade said the previous highs for May included 24.6C in Auckland in 2011, 22.4C in Hamilton in 2004, and a staggering 27.3C in Christchurch in 1978.
Philip Duncan from WeatherWatch said the outlook for the rest of winter was similar to last year.
But this year is likely to end with rain and warmth common in La Nina rather than the windier, dryer El Nino.
"We'll transition away from El Nino into neutral as we go through autumn and early winter," Duncan said.
"By the time we reach spring, we may be heading towards La Nina."
Niwa scientist Chris Brandolino said the outlook for winter was for above-average temperatures but average or below average rainfall.
"The next three months are going to be warmer but that isn't to say there aren't going to be cold snaps or frosts," Brandolino said.
Duncan said it was likely there would be more westerlies for the South Island, which meant the wet weather the west coast has been having will continue and Canterbury's drought may not end.
"We can have anything thrown at us because we are two islands in the Roaring Forties," Duncan said.