The shark alarm sounded at Coogee Beach in Sydney today, sending swimmers racing for the shore.
The alarm rang out at about 7am, with lifesavers announcing there had been "several" reports of a bull shark being spotted in the area.
Lifesavers were seen on jetskis patrolling the area, looking for the shark.
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Bull sharks are believed to have been involved in a recent spate of attacks in NSW waters, including one on 12-year-old Nico Antic, who remains in a critical condition in hospital.
Experts say bull sharks are likely to have been drawn to shallower, beachside waters after recent heavy rain in NSW, which has also churned up the waters to help conceal them.
Professor Jodie Rummer from James Cook University said bull sharks are one of the few shark species that can tolerate the brackish post-storm water, when saltwater and freshwater mingled more than normal.
Other experts say they are drawn to stirred-up areas to feed, as the rain flushes animals downriver.
People are urged to steer clear of murky waters after heavy rain.
