May was warmest on record - Niwa

Balmy temperatures and sub-tropical conditions saw New Zealand experience its warmest May on record, in a month that included floods, storms and a tornado.

Data from climate agency Niwa shows the month was almost 2.5 degrees Celcius warmer than usual, with rainfall double normal levels.

The figures won't be official until tomorrow morning, but principal climate scientists James Renwick said the provisional numbers were extraordinary.

"Two-point-five degrees doesn't sound like much, but for the average over the whole month that's huge," Renwick said.

"Normally 0.5 of a degree is a record-breaker."

The average monthly temperature had been 13.1C, a temperature normally expected for April, Renwick said.

The previous hottest May, recorded in 2007, had a mean temperature of 12.4C.

Rainfall totals were also extreme, especially in the eastern Bay of Plenty and Nelson regions.

The rain gauge at Whakatane airport showed the region had experienced 2.5 times its normal rainfall, while at Nelson airport, 3.5 times the normal levels were recorded.

Both regions had suffered heavy flooding during May, with residents evacuated from their homes in coastal areas of the Bay of Plenty mid-month due to slips closing roads.

A number of Nelson homes were evacuated last week when rivers burst their banks and threatened to inundate homes.

The wet, warm weather was caused by the predominance of northwesterly winds during the month, Renwick said.

A tornado in Auckland would have been caused by the same weather pattern, where warm and cold air collided during a storm.

Niwa's winter predictions will be forecast this week, however, Renwick warned the warm weather may be over for now.

"I would say that was the final fling of a warm spring and autumn," he said.