Swimmers, surfers and rock fishermen have been warned of dangerous conditions along the NSW coast on Friday and Saturday.

All NSW beaches should be considered "closed" unless flags are up, and only major metropolitan beaches are patrolled at this time of year, Matt Miller of Surf Life Saving NSW said.

"For most beaches ... the wind and swell is quite dangerous," he said.

"Surf Life Saving NSW is warning swimmers across Sydney and NSW that beach conditions are still dangerous at this stage and we'd advise people swimming, surfing and rock fishing to stay away from the beach at least until the weekend when conditions will start to abate."

A long inter-wave period averaging 11.4 seconds means incoming waves are "often a lot more powerful as they come in", said Sarah Chadwick, senior forecaster at the Bureau of Meteorology.

While east coast lows are common in August, swells greater than 2.5 metres occur only about 10 per cent of the time, Ms Chadwick said. 

Those conditions were expected to last until Saturday morning.