IF YOU'RE planning to go rock fishing today, Surf Life Saving NSW has one word for you: Don't.

Big surf is expected to pummel the state's coastline throughout the day, with fierce winds whipping up a four-metre southerly swell.

South-facing beaches and rock ledges will cop the worst of it, with Surf Life Saving NSW saying there will probably be long intervals between the waves.

"These long period swells means that a rock ledge might look safe for 20 minutes, but then a set wave will roll through and clean it out," lifesaving manager Dean Storey said.

"People always need to take extreme care when rock fishing, but during times of heightened risk they should avoid it altogether."

The Recreational Fishing Alliance also warned rock fishers to keep their rods under wraps for the next couple of days.

"Large waves can appear and wash over a rock shelf without warning, sweeping any persons in the way into the sea," the RFA's Stan Konstantaras added.