National Institute of Water and Atmospheric Research statistics released yesterday shows its Musselburgh station recorded a maximum temperature of 25.7degC, the highest since records began at that station in 1947.
Dunedin Airport reached its hottest daily maximum temperature on record as well - 26.5degC on May 4.
Lauder, Ranfurly and Nugget Point also recorded their hottest May day ever.
Farmers around the region finally got rain in the last fortnight of May after enduring months of drought.
Warmer-than-normal mean temperatures and a lack of rainfall had a major effect on feed and stock numbers.
Wanaka, Lauder and Ranfurly all broke mean minimum temperature records, with none getting below 3.2degC throughout May.
Niwa National Climate Centre principal scientist Chris Brandolino said a cool southwest low combined with a warm northeast high worked like "cogs'' to produce a warm northwest air flow responsible for the high temperatures.
A warming Tasman Sea also had input, he said.
"It's the persistent nature of it. The two [northwesters and warm seas] combine to produce the warm temperatures. It makes May 2016 the warmest on record.''
Nationwide, the mean average temperature across the country was 12.9degC, 2.1degC above the 30-year average from 1981 to 2010.
As well as that, the first five months of this year had all been warmer the normal.
