Beaches across the Illawarra, Newcastle, Wyong and the northern beaches from Palm Beach through to Freshwater have been closed.

Clovelly, Coogee and Maroubra have all been shut, along with Cronulla, North Cronulla, Elouera and Wanda, due to the dangerous conditions. The closures in the Sutherland Shire will be reassessed at 7am on Saturday.

Manly's North Steyne and Queenscliff have also been closed.

While many have viewed the conditions as equivalent to a surfer's paradise, Surf Life Saving NSW have said all beachgoers should heed warnings.

"It's pretty big and hazardous. Even experienced surfers can get caught out, we can't stop anyone going in but we advise all people to stay out of the water at this time," a Surf Life Saving NSW spokeswoman said.

Bondi Beach and Manly's South Steyne remained open at lunchtime on Friday.

The swell conditions have been generated by a deep low pressure system, and are said to be as big as Sydney is likely to see in January.

"I'm sure there are lots of surfers who will be pretty keen on it today," Bureau of Meteorology forecaster Tim Constable said.

"With south to south-west winds blowing from the land it means they are offshore. So as well as these decent waves the conditions are going to be pretty glassy."

Huge swell rolling in this morning #bondi #noswim pic.twitter.com/xMuBOcF47n — Bronwen Thompson (@bondigal) January 29, 2015

Mr Constable said that, while conditions are perfect for experienced surfers, for general-public swimming it could be quite misleading.

"The surface of the ocean can look quite smooth. You could look at the water for 10 or 15 minutes and not see any waves and then a huge wave can arrive."

The swells have been expected all week, with the deepening of a trough over the central Tasman Sea earlier this week.

Coastalwatch chief forecaster Ben Macartney said that, as far as east coast lows go, "it's a monster".

"It might move fairly quickly towards Tasmania, which may limit the duration of the swell. But some of the forecasts have it hovering over the south central Tasman Sea, which could maintain the swell on Saturday before easing on Sunday, Monday," he said.

Swells are set to be even more energetic along the south coast on Friday, reaching maximum heights of six metres at Port Kembla and even up to nine metres at Eden.

Wollongong can expect maximum swells to reach about four metres.

"Sydney can expect sunshine for most of the day and it should reach a maximum of about 27 degrees," Mr Constable said.