Earlier this year, scientists were alarmed to discover millions of sea sponges there had turned from velvet-brown to bone-white – making for one of the worst bleaching events ever documented among sponge species anywhere.
Even throughout winter, our seas have been running abnormally warm - surface temperatures over July ranged from 0.5C to 1.3C above average – and this trend was only expected to continue into spring and summer.
Scientists measure and define marine heatwaves in different ways.
Under a definition used by Niwa, marine heatwave conditions are reached when sea surface temperatures exceed the 90th percentile of the long-term average for at least five days in a row.
Once established, they're also categorised by intensity.
A freak event that engulfed New Zealand over the summer of 2017-18 was defined as "severe" - a threshold since reached in pockets of our coastal waters.
While causing dramatic effects in oceans – the 2017-18 event was linked to cascading losses in mussel and kelp beds, and also drove tropical fish into colder climes – marine heatwaves pushed up temperatures on land as well.
The summer of 2017-18 proved New Zealand's hottest, bringing scorching days and nights, early grape harvests and widespread glacier melt.
The Moana Project's manager, Dr Malene Felsing, outlined how a wealth of new data had brought warming patterns in our seas into sharper focus.
"We have over 250 sensors attached to commercial fishing gear, from inshore cray potters to deepwater trawlers in the Southern Ocean, sending back ocean temperature and depth data," she said.
"These sensors collected an astounding 4.3 million observations last year, 700,000 alone in the month of June.
"Altogether, the sensors have been collecting underwater data for a combined time of over 12 years.
"In 2020 New Zealand had almost no real-time observations of coastal ocean temperatures, and now we have millions – thanks to the fishing industry."
The ocean temperature data collected would be incorporated into MetService ocean forecasts, improving accuracy.
"From the more accurate models we get better forecasts, including for marine heatwaves, and this is vital information that will help us prepare for and mitigate climate change impacts," Felsing said.
"Fishers get their data back, so they know exactly what the temperature was where they fished.
"This, along with the improved forecast and warning of marine heatwaves, is the first step in helping fishing and aquaculture industries better prepare for ocean warming."
Research carried out as part of the project indicated that average sea temperatures could increase by 1.4C by 2060 - and almost 3C by the end of the century.
"This has wide implications for marine life, including fisheries and aquaculture," Felsing said.
"This means that by mid-century we may be facing 260 days of marine heatwaves per year, increasing to 350 days by 2100."
But already, the picture was remarkable.
Niwa forecaster Ben Noll said many parts of New Zealand – spread from the Coromandel Peninsula to areas off North Canterbury – had essentially been in a marine heatwave state since summer.
"A marine heatwave that's short-lived is one thing – but one that's been persistent as this is another."
Noll said he wouldn't be surprised if New Zealand's winter sea temperatures went down as the warmest in the books.
"It does make me wonder what this compounding effect of month-on-month marine heatwave conditions means for our marine life."
Warmer seas were partly to blame for recent bouts of extreme rainfall, including this week's deluge.
Tropical regions currently experiencing the biggest marine heatwaves on the planet had sourced a series of recent "atmospheric rivers" snaking down to New Zealand.
On top of that, warmer coastal sea temperatures made storms worse by adding energy to low-pressure systems as they passed over water.
"With the effects of La Niña and a warming climate giving things an extra push, the impact now being realised here in New Zealand is not just winter warmth – but copious amounts of moisture and flooding."
