Coastal waters around the country became unusually warm last month, NIWA says.
November sea surface temperatures were 1.1C to 1.8C warmer than average, with the north and west of both islands observing their warmest sea surface temperatures on record - since at least 1981.
NIWA meteorologist Ben Noll warned marine heatwave conditions could continue into the new year.
Marine heatwaves are classed as periods of unusually high sea surface temperatures for more than five days. In parts of the Tasman Sea near the west coast of both islands, a marine heatwave has been occurring for more than six months. More localised marine heatwaves have been occurring for several months near Northland, Auckland, Bay of Plenty and Fiordland.
"2021 and 2017 saw two of NZ's most significant marine heatwaves, and we're tracking warmer than at the same point in those years.
"Although we are expecting a cooler start to December, there's an indication that temperatures may become more unusually warm again during the second half of December into January," Noll said.
NIWA's principal scientist marine ecology Vonda Cummings warned "marine heatwaves can have concerning effects, with heat stress pushing marine organisms towards or beyond their thermal tolerance limits.
"These events can upset the balance of marine food webs and disrupt ecosystems."
NIWA's research shows that marine heatwaves are going to grow longer and hotter as a consequence of human-made climate change.
Marine heatwaves are twice as frequent now compared to 30 years ago, with the average ocean temperature climbing 1.5C over the last century.
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