Parts of Auckland, Tauranga, Gisborne and Napier have already recorded over a year's worth of rain in the first six months of 2023.
NIWA said many of its rainfall measuring locations in the northern and eastern North Island had already recorded their wettest first half of the year on record.
Eleven locations across four regions recorded over a year’s worth of rain between January and June.
The agency's meteorologist Ben Noll said the results probably won't come as a surprise.
"Those living in Northland, Auckland, the Coromandel Peninsula, Gisborne, and Hawke’s Bay have dealt with a constant barrage of sub-tropical lows, atmospheric rivers, and ex-tropical cyclones, which caused copious amounts of rainfall. It has been quite relentless."
The locations that recorded over a year’s worth of rain in the first six months of 2023 were: Kaikohe, Whangārei, Warkworth, Whenuapai, Tauranga, Albany/North Shore, Western Springs, Leigh, Gisborne, Māngere, Whangaparāoa, and Napier.
On the rainiest end of the spectrum, Kaikohe in Northland received over 130% of its normal annual rainfall from January to June, Noll said.
Meanwhile, on the driest end of the spectrum, Waimate in South Canterbury received just 33% of its normal annual rainfall in the same period.
It was also a warm start to the year, with a temperature 1.1 degrees above average. This is the second warmest such period on record, according to NIWA.
Why the wet and warm summer? What's to come?
According to NIWA, the lingering influence of La Niña, Southern Annular Mode, marine heatwaves, and climate change were behind the abnormal conditions.
El Niño is emerging in the tropical Pacific and is expected to bring notably different weather patterns to the country during the back half of the year as compared to the first half, according to NIWA.
"During late winter, spring, and summer, southwesterly-to-westerly winds will become more prominent. Historically, this has increased the chance for drier-than-normal conditions in eastern areas of the country and caused more rain in the west," Noll said.
"In summary, a change in the climate driver means a change in the wind, and ultimately, a likely change in rainfall patterns. The weather is likely to be quite different to what we’ve been living through in recent times."
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