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.
"I've seen them change their diet ... as we've thinned out all the fish. They've changed to dolphins, turtle, dugong," he said.
"They won't stop eating. They'll just change their diet when the fish get thinned out."
"They're coming in closer," he told Southern Cross Austereo.
Ballina Mayor David Wright said one surfer had recognised the shark threat on Sunday evening and got out of the water.
"He said he got out because there were baitfish, there were lots of fish going close to the edge of the water and sharks actually feed on that," he told ABC Radio on Tuesday.
The surfers who were with Mr Nakahara said the shark was three to four metres long.
"It came up behind and took his board and his legs," he said.
Rescuers took Mr Nakahara to shore and performed CPR but he died on the beach.
NSW Police are assessing when to reopen the beaches.
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