Only a handful of beaches across the city remained open, including Clovelly, Bondi and Manly - however Randwick City Council advised only experienced swimmers to swim at Clovelly due to the large swell.

Northern Beaches Council issued an alert that all beaches except for Manly were closed due to "extremely hazardous conditions today, swell heights in the four-to-five-metre range and a gale wind warning".

The Bureau of Meteorology issued a severe weather warning for damaging surf right along the coast of NSW on Tuesday and "strong wind warnings" are again in place for Wednesday.

The bureau warned beach conditions from Sydney to Byron Bay could be dangerous, and people should stay well away from the surf and surf-exposed areas.

Weatherzone meteorologist Kim Westcott said the dangerous surf was caused by a low-pressure offshore system, and was mainly affecting south-east facing beaches.

Surf Life Saving NSW spokesman Liam Howitt said the heavy surf conditions built overnight.

"Most of the NSW coastline is experiencing quite large surf today, looking at peaks in excess of three-and-a-half metres so it's quite significant," he said.

"It looks like it's going to continue for at least the next few days."

Almost all beaches in the Sydney, Hunter, Central Coast, Lake Macquarie and Illawarra regions were closed on Tuesday due to the dangerous surf.

Mr Howitt said beaches would all be affected differently and people should check the conditions.

The rough surf was a reminder for people to only swim at patrolled beaches.

"There's a number of beaches that are patrolled by council lifeguards at this time, right across metropolitan Sydney but also in other areas of NSW in particular in the holiday destinations," he said.

"We strongly encourage people to make the extra effort and swim at those beaches."

Mr Howitt said during this weather it was important for rock fishers to take extra care, particularly when getting on and off platforms.

"We're looking at an additional swell period of 13 seconds, which is quite powerful," Mr Howitt said.

"It means the waves are going to be packing a punch when they do hit the headland.

"We've had a number of rock fishing fatalities this season already and the data's clear and it's consistent, and it's that lifejackets do save lives."???