A powerful winter swell is causing large waves along the NSW coastline at the start of this week.

While a high pressure system is currently causing settled weather over eastern Australia, seas have been anything but calm during the last 48 hours.

A southerly swell generated by a low pressure system passing over the southern Tasman Sea on the weekend is now travelling up the NSW coast.

Maximum wave heights reached close to nine metres off the coast of Sydney's Northern Beaches on Monday and around six metres off Port Kembla and Crowdy Head.

The large southerly swell coincided with offshore winds during Monday and Tuesday morning, delivering clean waves for the few brave and experienced surfers that ventured into the water.

A hazardous surf warning is in place along the NSW coast from the Illawarra up to the Queensland border today.

Wave heights will drop on Wednesday as the southerly swell loses energy and seas will be notably flatter by Thursday.