A rare 22 second wave period was detected off the coast of Sydney on Tuesday morning as an energetic long-range swell produced by ferocious polar winds reached the NSW coastline.

Waves in the ocean are typically measured by their height and period:

• Wave height refers to the vertical distance between the wave crest and the preceding wave trough.
• Wave period refers to the amount of time between the wave crests (or troughs) of two consecutive waves. The distance between two wave crests (or troughs) is called the wavelength.

Wave period is an important part of wave dynamics because the longer the period (time difference between waves), the greater the amount of energy associated with the swell.

Along the coast near Sydney, it is common to see wave periods around 8 to 12 seconds, while periods above 14 seconds are rarer. According to data from the Manly Hydraulics Laboratory, wave periods exceeding 16 seconds have a 0.8% likelihood of occurring around this time of year, while periods above 20 seconds have a 0.004 % chance of occurring each year.

On Tuesday morning, a waverider buoy located about 12 km east of Freshwater Beach registered a wave period of 22 seconds. This is among the largest wave periods ever recorded off the coast of Sydney by the Manly Hydraulics Laboratory, with data available back to 1992.

Image: Directional spectrum wave observations at 2am AEDT on Tuesday for a waverider buoy located near Sydney, showing a wave period of 22.2 seconds. Source: Manly Hydraulics Laboratory

Tuesday morning’s exceptionally long wave period is occurring in a swell that has relatively small wave heights of around one metre. Fortunately, this means the energetic swell isn’t likely to cause damage along the coast. However, it may still cause surges of water that ebb and flow along the coast, possibly causing boats to drag on their moorings and anchors.

While it may seem odd to have such a long period swell impacting eastern NSW during a fairly benign week of weather, these energetic groundswells are always produced by weather systems a long way from the coast.

In this case, the swell surging along the NSW coast on Tuesday was caused by powerful winds blowing near Antarctica last week. The resulting swell from these ferocious polar winds travelled more than 4,000 km across the Southern Ocean and Tasman Sea before hitting NSW.

Image: Visible satellite images captured on Friday, March 21, 2025, showing a polar low to the south of New Zealand. This low caused the swell that reached eastern NSW on Tuesday, March 25. Source: NASA Worldview / Terra-MODIS

This exceptionally long period groundswell will continue to spread north through the Coral Sea in the coming days and could fill into the Solomon Islands by Thursday.