A buoy 400 miles off the coast of New Zealand in the Southern Ocean has detected the biggest wave ever measured in the southern hemisphere, at 19.4m high.

The wave was just a shade smaller than the White House, which stands at 21m high. Rogue ocean waves are surprisingly common, and are caused in part by sudden changes in wind speed.

"This is one of the largest waves recorded in the southern hemisphere," said Tom Durrant, senior oceanographer at MetOcean, which recorded the wave, in a statement.

The buoy is measuring waves in the Southern Ocean to help design better ships for the Royal New Zealand Navy.

"There's a possibility we'll get something even bigger, and we are starting to get towards the biggest waves ever measured anywhere," David Johnson, the technical director of MetOcean, told Stuff NZ.

"The main excitement is, basically, we're measuring something in a place that's never really been measured before, and we're capturing some quite exceptionally large waves."

Ship at sea
HMNZS WELLINGTON encounters rough waters in the seas around Antarctica. The largest wave ever recorded in the southern hemisphere was 19.4m high New Zealand Defence Force

Although this is the tallest wave ever recorded in the southern hemisphere, the north has seen some wilder waves. In November 2007, a staggering 21m-high wave was recorded by sensors in the North Sea, making it one of the most steep-sided waves ever recorded. In terms of sheer height, a 27m wave was measured off the coast of Scotland in 2013.

The Southern Ocean is one of the roughest in the world, and has an important role in mixing water masses in the world's oceans. It has some of the coldest waters in the world, which are churned up by underwater mountains.

Even with these inhospitable conditions, life thrives in the Southern Ocean. It is a biodiversity hotspot, with incredibly tough sponges, sea spiders, sea cucumbers and starfish, as well as iconic penguins and killer whales.