Did Cyclone Cook really make a monster 12m wave?

Waves crash on to Mount Maunganui beach as Cyclone Cook moves in last night. Photo / John Borren

Did a wave the height of a four-storey building really form off the Bay of Plenty?

A 12m-high reading that was picked up by a wave buoy amid the thick of Cyclone Cook was one of the more dramatic figures that have emerged from the storm.

The regional council-operated buoy, located 13km off Pukehina Beach at a spot where the water is 62m deep, lies in a central position within the curve of the Bay of Plenty.

A graph shows maximum wave height climb sharply from around 5.5m yesterday afternoon to nearly 12.5m at 6pm, just when the cyclone was beginning to make landfall between Tauranga and Whakatane.

The "maximum wave height" simply measures the largest wave recorded, from peak to trough.

The instrument's "significant wave height" readings - the average measurement of the largest third of waves, something which corresponds well to our own visual estimates of wave height - dramatically climbed from 2.5m to 8m at 6pm.

At around 6pm, the average wave height was 5.1m.

But did one really reach over 12m?

"It's in the realms of possibility," said Niwa forecaster Chris Brandolino, who formerly served as a marine forecaster with Australia's Bureau of Meteorology.

"Could it have been bad data? I suppose ... but I'm inclined to believe that it was good data.

"So if we assume that it's good data, you can find waves can be above the height of the significant wave height.

"My understanding was that the buoy was continuously reading 8m, so if we take that as the general wave height, 12m falls within double - and these things can happen."

For such a monster to have been created, Brandolino said a combination of three factors were needed: a high wind speed, a large area of water for winds to blow across, or what's called fetch, and a persistent wind direction.

"If the winds are coming from the north, continuously, the waves will build, but if it switches to the west, just by 30 degrees, a new wave will build: so you'd want minimal change in direction."

What would have it been like to confront a 12m wave?

"I would say it could have capsized a boat ... depending on the boat."

According to the World Meteorological Agency, it was also a buoy that last year recorded the highest wave ever: a 19m-high beast rolling in the North Atlantic Ocean between Iceland and the UK.