People living around Wellington's south coast have been warned to be on alert for massive waves that could reach higher than 6m tomorrow night, and potentially cause local flooding and damage.
In Wellington, MetService has issued a heavy swell warning for Baring Head to Sinclair Head from 6am tomorrow to 11pm Wednesday, as a cold air mass from the Antarctic ice shelf sweeps into the country.
Wellington Region Emergency Management has warned that heavy swells will cause problems around many areas over the next few days – particularly the Wellington south coast.
"People need to be aware that southerly swells are forecast to rise to 6m Tuesday morning easing a little Wednesday evening," the agency advised.
Niwa has also predicted large waves hitting the south coast tomorrow night – potentially reaching as high as 6.7m around 9pm tomorrow.
Over tomorrow and Wednesday, waves could reach as high as 7m at southern areas of Banks Peninsula in Canterbury, 5.5m at the Kaikoura coast, and 5m at Cape Turnagain in Hawke's Bay.
"These are all quite significant swells," MetService forecaster Peter Little said.
"And for Wellington, particularly, you're also going to have very strong southerly winds, so it's going to be pretty brutal on the south coast."
Even for windy Wellington, he added, the wave heights were unusually big.
What did that mean for seaside residents?
"I guess whatever they can do to try to mitigate waves crashing into garages and that type of thing," he said.
"I'm sure they'd know well what to do from last time."
In April last year, amid the national Covid-19 lockdown, five homes in Owhiro Bay had to be evacuated – and one person was taken to hospital - as monster waves and high winds swept into the coast.
"People just need to be safe and stay away from the coastline when these waves are coming in, because they'll potentially be quite dangerous."
Little explained a major factor increasing the waves' height was something called fetch – or the distance of ocean the waves were driven along by strong winds.
"When the wind is blowing, these waves gradually add up over time – and we've got a fetch south of the country, which is very, very long," he said.
"If you look at the isobars [lines on meteorological charts connecting points of equal atmospheric pressure], to the south of the country, they look a little bit like train tracks, and are very straight.
"So these waves are having a big opportunity of building and building as they come up towards New Zealand."
As well as height, the waves were also packing plenty of power.
Little pointed to their "wave period" – or the distance between two waves passing through a stationary point, measured in seconds.
"The longer the separation between the waves, the more energy they contain – and some of these waves have about a 15-second period, which is getting quite long," he said.
"This means that when they come up on to beaches, they can run up further. And if they hit any structures of sea walls, they contain more energy and can therefore damage them more effectively."
The waves were also coming as New Zealand moved out of a king tide period – or especially high tides caused by the fuller moon and the close orbit of the moon to Earth.
"There's been a fairly full moon recently, so that doesn't help much either," Little said.
"The fact that this event is going through pretty much most of tomorrow and Wednesday, for somewhere like Wellington, there'll be at least three or four high tides during this period of large waves.
"So it's not just one risk around one particular high tide: there will actually be multiple risks."
Niwa Weather forecaster Chris Brandolino said the heightened risk of inundation meant locals should be closely following emergency management advice.
"I'm not saying it's a guarantee of coastal inundation – but this [weather system] elevates the chances of it actually happening."
With southerly gales and heavy swells forecast in the Cook Strait, sailings of the Interislander tomorrow have also been cancelled.
"At this stage it is likely that passenger sailings on Wednesday will also be cancelled, although some freight sailings that day may be possible," a KiwiRail spokesperson said.
"We are reviewing the situation constantly, but we are about to get some very rough weather through the strait, and we will not resume sailing until we are confident we can do so safely."
The Kaiarahi 10.30pm sailing from Picton tonight would be the last sailing before the cancellations come into effect.
Away from the mainland, Little said wave heights near the Chatham Islands could reach between 8m and 10m tomorrow and Wednesday.
Deeper below New Zealand, in wild and windswept Southern Ocean stretching across notorious latitudes dubbed the "roaring 40s", "furious 50s" and "screaming 60s", waves were likely to be much larger.
During the depths of winter, these waves are enormous, averaging more than 5m, regularly exceeding 10m, and sometimes likely reaching more than 25m - or the equivalent height of 16 cars stacked on top of each other.
Already this year, one operating wave buoy drifting to the southeast of the country recorded a wave with a maximum wave height of 16.3m.
Anything more than 20m high is considered highly hazardous to vessels. Waves that climbed to 14m forced the HMNZS Wellington to turn around part-way to the Subantarctic islands in 2014.
And scientists predict these waves will grow all the larger as the planet warms.
One recent study found that extreme waves in the ocean had grown by 30cm - or 5 per cent - in just the past three decades, all while the region had grown stormier, and even gustier, with extreme winds strengthening by 1.5m a second.
Another found that a warming planet will cause stronger storm winds triggering larger and more frequent extreme waves over the next 80 years - with the largest increases occurring in the Southern Ocean.