The National Institute of Water and Atomospheric Research says last month was the warmest August since records began more than 150 years ago.
The average temperature was 10.4 degrees celsius, with the highest reading recorded at 25.5°C degrees in the Auckland suburb of Henderson.
Of the main centres, Auckland and Tauranga were the warmest, while Christchurch was the coolest.
NIWA says temperatures on average were about 2°C higher than usual.
Warmer weather is also expected for September.