New Zealand has reported its hottest June on record as temperatures continued to soar in a pattern consistent with global warming, a government scientific agency has said.

It is the winter season in the southern hemisphere at present.

Daily temperatures in New Zealand in June averaged 10.6C - this is 1.9C above average despite a cold snap late in the month, the National Institute of Water and Atmospheric Research (NIWA) said.

"It's huge. There's only been 13 times since 1909 that we've had such an anomaly," NIWA scientist Chris Brandolino told public radio.

"What's alarming is that over the past 10 years that's happened six times."

Mr Brandolino said that while shorter-term factors such as warmer ocean temperatures helped heat the air over New Zealand, climate change was a constant underlying factor.

"We've had more north-easterly air flows than normal (from the Pacific), so they're coming from a warm place, and ocean temperatures are also warmer than average," he said.

"Then you throw in the background of climate change and it adds up to a record warm month for June."

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New Zealand had its seventh-hottest year in 2020, which was also the seventh year in a decade that temperatures have been in the top 10 highest ever recorded.

The country's centre-left Prime Minister Jacinda Ardern declared a "climate emergency" last year, saying urgent action was needed for the sake of future generations.

New Zealand has committed to becoming carbon neutral by 2050 and also vowed to generate all its energy from renewable sources by 2035.