The current weather conditions are brewing up a perfect storm for not one but two species of shark in the water off Sydney's beaches.
Shark expert Dr Chris Pepin-Neff told Today that the combined factors of recent rain and sea temperatures mean both bull sharks and great whites are being lured closer to shore.
Pepin-Neff said sea surface temperatures of 20 degrees and above were ideal for bull sharks, while 20 degrees and below was perfect for great whites.
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And right now, "we're right in the middle".
"It's the end of May, we're right at that seasonal point," he said.
Added to that, recent heavy rain had stirred up brackish water closer to the shore, which also made ideal hunting conditions for bull sharks.
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And great whites were being drawn in closer by the cooling temperatures.
"I want surfers to be aware if you're in the Northern Beaches, if you're doing surfing right now, white sharks are in closer," Pepin-Neff said.
He urged surfers to stick together in groups in the water for safety's sake.
Other swimmers were told to stay out of brackish water, and if they were swimming at an ocean beach, to stay between the flags.
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Pepin-Neff said the noise generated in the water by a group of people could intimidate sharks and keep them at a distance, so there's safety in concentrated numbers.
Sydney recently saw a spate of attacks believed to have been carried out by bull sharks, in January this year.
Among the victims was 12-year-old Nico Antic, who was fatally mauled and died in hospital.
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