Meteorologists have detected a highly unusually phenomenon off the coast of Sydney.

A long period between waves has been detected, at a time when the likelihood is smaller than the chance of being struck by lightning in Australia.

Recorded 12km off the coast of Freshwater Beach this morning, the wave period – the time that passes between consecutive two wave crests hitting a certain point – was 22 seconds long, almost triple the expected length at this time of year, according to Weatherzone.

Longer wave periods, known as groundswells, typically cause more powerful waves, with common periods off the coast of Sydney between 8 and 12 seconds.

Wave periods exceeding 16 seconds are 0.8 per cent likely to occur at the moment, while periods above 20 seconds have a 0.004 per cent chance of occurring each year, according to data from the Many Hydraulics Laboratory.

Compare that to the 1 in 12,000, or 0.0833 per cent, chance of being struck by lightning.

The wave period is among the largest ever recorded by the Manly laboratory, whose records date back to 1992.

Meteorologists say that it was caused by powerful winds in Antarctica last week, with a swell surging along the NSW coast.

As the wave period happened in waters with relatively short wave heights, it shouldn’t cause too much harm to the coastline, though it may lead to surges with ripple-on effects, causing boats to drag their moorings and anchors, Weatherzone said.