Weather experts are monitoring a looming marine heatwave that will be welcomed by beachgoers - but poses dangers for our marine ecosystem.
Meteorologists are keeping a close watch on coastal waters that are again running unusually warm nationwide - with the mid-summer peak still weeks away.
Water safety advocates have already sounded a warning that the high number of people enjoying the warmer water could lead to more dramas along our coastlines, as Kiwis flock to the beach as daytime highs top 25C.
"We are asking if people go rock fishing that they wear a life jacket and tell someone where they are going," Surf Life Saving Northern region head of operations Alan Gibson said.
"If they are bathing, just to never swim alone and if you get caught in a rip to ride it out and put their hand in the air and seek lifeguard assistance."
Bystanders who witness people in strife in the water should dial 111 or get the attention of lifeguards if they were on a patrolled shoreline, Gibson said.
"Don't put yourself at risk and dial 111 and ask for Coastguard and Surf Life Saving," he said.
"If it is a patrol location, put up your hand and alert the lifeguards. If you enter the water, do try and bring some sort of floatation device with you.
"Sadly, in New Zealand, a lot of bystander rescues end up in a double or single tragedy for the person who is entering the water to save someone else."
The warming coastal waters comes after "marine heatwave" conditions formed in some regions in the season's lead-up, before sea surface temperatures cooled back to normal last month.
Unusually warmer seas are a classic calling card of a strong La Nina system that's now influencing our weather - but also of a warming planet.
It's now been nearly 50 consecutive months since New Zealand had a month with below average temperatures.
Our coastal waters have also warmed at a rate of 0.2C per decade since the start of the 1980s - notably off the South Island's west coast, and east of Wairarapa's coast.
The latest measurements showed seas in large stretches of coast have warmed again - and may well keep heating up, Niwa forecaster Ben Noll said.
Marine heatwaves have been known to melt glaciers, send tropical fish into colder climes and even influence harvest seasons in orchards and vineyards.
While ideal for beach-goers, they're potentially devastating for marine ecosystems, and are expected to grow stronger, longer and more frequent under climate change.
Since the start of the year, temperatures in the north of the North Island, along the west and east of the South Island, had risen to 0.6C above average.
"Five out of six climate regions we monitor now have an anomaly that's sitting above zero at this point," said Noll, adding seas in the west of the North Island were currently cooler at 0.1C below average.
He said the 0.6C anomaly had become something of a new baseline since an intense and unprecedented marine heatwave over the record-hot summer of 2017-18.
Waters of the Tasman Sea ultimately warmed, on average, by 3.7C above normal to reach 20.6C - driving kelp die-offs and heating water to the point some surfers swapped wetsuits for board-shorts.
That event was followed by a second heatwave event the following summer, before the potential for the third one in four years formed up over spring.
"Right now, it's not like people will be going into the water and thinking, wow, it's really warm," Noll said.
"But that said, the upcoming pattern with warmth that we expect, with abundant sunshine in the North Island, will probably be having a significant impact on day-to-day ocean temperatures.
"In the upper North Island, especially, I'd imagine water over the next few days is going to be quite warm because of this short-term pattern."
Over the rest of summer, Noll said coastal waters could generally be expected to warm rather than cool.
"Other than perhaps the lower and eastern South Island, which has been a little more exposed to cold snaps, much of the country should expect to experience above-average air temperatures, and that should come with above-average sea temperatures."
These typically peaked around late January and early February.
"We're seeing a bit of variability at the moment - but over the longer term, with that warmth, we'll probably see [sea temperatures] creep up later in the month."
Noll said the summery weather much of the country has enjoyed this week is set to take a colder turn.
After a spell of more warmth today - Gisborne could see a high of 31C, and Hanmer Springs 28C - a southerly change was forecast to sweep up the country.
Showers were on the cards for most of New Zealand later today - with some potentially heavy and thundery around the lower South Island.
"As we look further ahead, things will turn quite a bit warmer as we go toward the middle of the month," Noll said.
"So we're looking at perhaps five to seven days of mixed temperatures around New Zealand - and then a warm trend smack dab in the middle of January."
Beach Safety Messages
1. Choose a lifeguarded beach and swim between the flags
2. Read and understand the safety signs - ask a lifeguard for advice as conditions can change regularly
3. Don't overestimate your ability or your children's ability to cope in the conditions
4. Always keep a very close eye on young children in or near the water - keep them within arm's reach at all times
5. Get a friend to swim with you - never swim or surf alone
6. Watch out for rip currents, they can carry you away from shore. If caught in a rip current, RELAX and float, RAISE your hand to signal for help, RIDE the rip until it stops and you can swim safely back to shore. Remember - nobody is stronger than a rip.
7. Be smart around rocks: When fishing, never turn your back towards the sea and always wear a lifejacket
8. If in doubt, stay out!
9. If you see someone in trouble, call 111 and ask for Police
10. Be sun smart – Slip, Slop, Slap and Wrap. Protect your skin and eyes from the sun's damaging rays
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