Twin ocean swells to deliver massive, dangerous surf this week

Large and hazardous surf is forecast to batter the NSW coastline over the coming days, as two powerful swell systems move in rapid succession across the Tasman Sea and Southern Ocean.

According to Weatherzone, the first swell arrived on Wednesday morning, generated by a deep Tasman low spinning between NSW and New Zealand.

Wave buoys recorded a sharp increase in swell height near Sydney shortly after sunrise, with significant wave heights jumping from about 1m to more than 2m within a matter of hours.

This system, combined with gale to storm-force winds over the Tasman, is expected to produce peak wave heights of 4.5 to 5m along the lower Mid North Coast, Hunter, Sydney, and Illawarra coastlines.

Maximum wave heights could exceed 10m in exposed offshore areas.

With peak wave periods reaching up to 15 seconds, forecasters warn of dangerous conditions for swimmers, surfers, and boaters, particularly on south-facing beaches.

The risk of coastal erosion is also high, intensified by recent heavy swell events and large high tides.

The first swell is expected to ease by Friday, with wave heights dropping to about 1.5 to 2m, but more powerful surf is on the way.

A second system, originating deep in the Southern Ocean, is forecast to arrive on Saturday, driven by storm-force winds more than 2500km south of Australia.

This long-period swell will bring deceptively strong waves to the NSW coast despite reduced heights of about 2m.

The second swell is forecast to have an extended wave period, peaking between 17 and 18 seconds, allowing powerful wave energy to reach up to 150m further inland along the shoreline.

Weatherzone warns this could cause abnormally large waves every five to 10 minutes, catching swimmers off guard in otherwise calm conditions.

The combination of extended wave periods and strong rip currents could make for hazardous conditions through the weekend, even at more sheltered beaches and bays.

Beachgoers are urged to follow advice from Surf Life Saving NSW and to avoid swimming at unpatrolled beaches during dangerous surf conditions.