Surfers should get out of the water if they see a lot of baitfish around them, shark experts say.

They believe warm water and runoff from rainfall is attracting small fish, which in turn attract sharks.

The warning comes after 41-year-old Tadashi Nakahara was killed by a shark that tore his legs off at Shelly Beach, near Ballina, northern NSW.

It was Australia's fourth fatal shark attack in five months.

Marine ecologist Dr Daniel Bucher says the shark, believed to be a great white, could have been following a food trail that led it close to the shoreline.

Dr Bucher, from Southern Cross University, says recent rainfall meant food for fish was washed out from rivers to the ocean, drawing them in.

"Don't swim if you know there are plenty of baitfish around, especially if they are breaking the surface," he said on Tuesday.

"It usually means something is chasing them from below."

Dr Bucher said dusk and dawn were notorious for shark activity and also warned swimmers to avoid river mouths and stormwater drains after rain.

Shark expert Vic Hislop said heavy fishing was thinning out the ocean, leading sharks to hunt for food close to shore.

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NSW fatal shark attack keeps beach closed