Whanganui recorded a maximum temperature of 28.1C on Tuesday, making it the hottest place in the country.
The high came after a nine-day dry spell, which ended with the arrival of rain overnight and continuing today.
Meteorologist Stephen Glassey said the last noticeable rain was recorded at Whanganui Airport on January 24.
"Whanganui could get up to 10mm of rain today before it clears in the afternoon."
Glassey said the rain, accompanied by strong winds, was because a low pressure system was moving up from the South Island.