Big enough to threaten ships and coastal roads, what scientists call "extreme waves" will likely become more common in the waters off New Zealand as the effects of climate change further set in.
In a new study, researchers at the University of Waikato and NIWA have trawled through nearly 50 years of records to gain a better picture of these ocean monsters.
Because they're very rare - and some occur only every 10 to 20 years - extreme waves are difficult for scientists to predict, and long datasets generally haven't been available to them to analyse.
But with NIWA having just completed a 45-year wave hindcast, Waikato University PhD student Victor Godoi and his fellow researchers saw a perfect opportunity to look at how the waves had appeared around our coasts.
"There have also been some recent high impact papers that show that climate change will cause an increase to storminess," Godoi said.
"This could have a big effect on the coastline, but to detect these changes, earlier we need to have a good baseline understanding of what the past state was like."
The biggest reason to learn more about them was the threat they posed to us: through surfing, sailing or fishing, or catching a ferry across the Cook Strait, many of us encountered waves each day.
Harbours, oil platforms and sea-side roads meant we had to be monitoring sea state every minute, and the larger wave, the bigger the threat.
"The energy in a wave scales with the square of wave height, so a five-metre wave is a lot more than five times as energetic than a one metre wave," Godoi said.
The team drew on several large models, including buoy data from a few locations around New Zealand, to hunt out trends and patterns in the data.
They found the largest extreme waves were generally found in southern New Zealand, while the smallest ones occurred in areas sheltered from southwesterly swells, such as the Hauraki Gulf and parts of the Cook Strait.
Although the number of extreme events anywhere varied throughout the year, their intensity was more consistent, Godoi said.
El Nino and La Nina climate patterns also had some influence: those stronger northeasterly winds that buffetted the north and northeast of New Zealand during La Nina could bring extreme waves, which likely explained why beaches eroded on northern beaches in La Nina events.
As for climate change, the study drew a link to an increase in of extreme wave events on our south and east coasts between 1958 and 2001 - a finding in line with an observed intensification of westerly winds in the Southern Ocean.
"We, of course, cannot determine whether these are an early sign of the effects of climate change, or simply due natural climatic cycles - we just happen in a period where the waves are higher," Godoi said.
"The Earth system is somewhat chaotic due to the large number of variables contributing to the development of weather and climate."
While this meant it was almost impossible for scientists to be 100 per cent in making highly-accurate, long-term forecasts, there was already much research indicating an intensification of winds, bringing larger waves, and sea level rise.
"Our study is consistent with this trend."