While it's made for ideal beach conditions, a "marine heatwave" that's engulfed the Tasman Sea may have been damaging for ocean species, scientists say.

The latest heatwave kicked off in November and has persisted for most of this month, pushing coastal water temperatures to more than 2C above average and growing to its most intense at the west of the country.

Climate scientist Dr Jim Salinger said, among other factors, La Nina-driven winds had forced a stronger East Australian Current, sending warmer waters further south into the Tasman Sea.

"The current marine heatwave will have had impacts on coastal and near-shore environments - and possibly on fisheries."

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Professor Simon Thrush, head of the University of Auckland's Institute of Marine Science, wasn't yet aware of any direct ecological impacts in New Zealand, but said extreme events could "profoundly" affect how marine ecosystems functioned and the species they contained.

"Often high temperature events combine with other stresses causing major changes," Thrush said.

"The type of things we could see this summer are shellfish die-offs and sick-looking kelp forests - in the past we have seen these changes not just because of higher temperatures alone, but in combination with disease and overfishing."

"We might expect to see more warm water species, especially in the north eastern part of New Zealand, so you might see some unusual species when you are snorkelling.

"We still have a lot to learn about how our marine ecosystems respond to warming, especially when combined effects are important, so keep an eye out for changes."

Salinger said an anomalously warm event became a marine heatwave if it lasted for five or more days, with temperatures warmer than the 90th percentile based on a 30-year historical baseline period.

The latest heatwave kicked off in November and has persisted for most of this month, pushing coastal water temperatures to more than 2C above average and growing to its most intense at the west of the country. Image / Supplied
The latest heatwave kicked off in November and has persisted for most of this month, pushing coastal water temperatures to more than 2C above average and growing to its most intense at the west of the country. Image / Supplied

Four had been recorded so far this century: in the northern Mediterranean in 2003; off Western Australia in 2011; in the northwest Atlantic in 2012; and off southeast Australia in 2015-2016.

Salinger, who specialised in marine heatwaves while at Australia's CSIRO, said 2011's "Ningaloo Nino" brought sea surface temperatures between 2-4C above normal for 10 weeks, along around 1000km of Australian coastline.

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At sites close to shore, scientists observed changes among some ocean species, including more tropical macroalgae and fish species, and fewer temperate species that preferred cooler waters.

"The biodiversity patterns of temperate seaweeds, sessile invertebrates, and demersal fish were significantly different after the warming event, which led to a reduction in the abundance of habitat-forming seaweeds and a subsequent shift from a poorly developed community structure towards more tropical fish communities."

There were more impacts recorded after the 2015-16 marine heatwave, which lasted just over 250 days and reached a maximum intensity of 2.9C above average, although these were limited to species that lived in shallow waters near the shore.

That included the outbreak in Tasmania of an oyster disease previously linked to warmer water.

"In addition, the warm water temperatures in southeast Tasmania reduced performance in cultured Atlantic salmon that limited supply to seafood markets," Salinger said.

"The warm temperatures were also associated with out-of-range observations of several fish species, including yellowtail kingfish, snapper, dusky morwong, mahi mahi, blue moki and moonlighter fish."

He expected the current marine heatwave to linger for some time yet.

Over the next two months, the atmosphere around New Zealand was forecast to be characterised by higher pressure than normal to the south and southeast of the country, and lower pressure than normal to the north.

This pressure pattern was expected to be associated with easterly to northeasterly flow anomalies - a pattern consistent with regional conditions typically observed during La Nina events, Salinger said.

Global warming simulations suggested these extreme weather events would become more frequent over coming decades.