By Saturday afternoon, the rain had set in and the Sydney catchment was soaked. Heavy storms dumped 35 millimetres over Parramatta in half an hour, filling the creeks and bays that feed Sydney harbour and coastal beaches with a turbid brown run-off.

For shark risk, says Professor Culum Brown, a specialist in fish behaviour at Macquarie University, it was a perfect storm.

A man was attacked and critically injured in Manly near the North Steyne Surf Life Saving Club on Monday.Credit: Kate Geraghty

Sydney’s water was now warm enough for bull sharks, which are drawn to the region in the summer months, as were swimmers escaping warm and humid weather. The run-off had stirred up nutrients and bait fish, and it had flushed bull sharks from the rivers and estuaries into the harbour, foreshores and beaches.

Bull sharks, explains Marcel Green, who leads the NSW Department of Primary Industries shark program, are opportunistic ambush predators. They do well in rivers and bays and at dusk and dawn when low visibility presents them with an advantage over their prey.

By the weekend, these were the conditions that extended up and down the coast, all through the day.

At 4.20pm on Sunday, a 12-year-old boy who jumped off rocks at Nielsen........

© The Sydney Morning Herald