National Institute of Water & Atmospheric Research (Niwa) meteorologist Ben Noll's report shows that January 2017 was the last time New Zealand experienced below average temperatures.
"It's the climate record that keeps getting broken," he said.
Noll said New Zealand's climate was strongly influenced by sea temperatures. From January to June this year, the ocean temperatures had been about 0.35 degrees above average.
Christchurch recorded the lowest mean temperature of the six main centres, at 7.4 degrees, a degree warmer than average. Niwa attributed this to higher-than-usual air pressure over the southwest Pacific region, which had caused more warm northeasterly winds in June.
Christchurch also the fourth-lowest total sunshine hours since 1930, with 106mm of total rainfall recorded.