Some of Australia’s most popular beaches are back on shark alert as heavy rain creates murky conditions.

Swimmers and surfers in NSW have been warned to watch out for aggressive bull sharks in the aftermath of a two-day drenching.

WATCH THE VIDEO ABOVE: Shark warning as brackish water conditions follow heavy rain.

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Four attacks occurred in the state at the start of the year, including one inside Sydney Harbour that took the life of a 12-year-old boy.

Recent downpours have created similar murky waters, prompting the state government to urge those going for a dip to be on high alert.

“Take the necessary steps to reduce your risk of a shark interaction,” Agriculture Minister Tara Moriarty said.

“If your local beach is closed ... we ask beach goers, swimmers and surfers to not enter the water for their own safety.”

More than 100mm of rain fell in parts of Sydney on Thursday, causing freshwater runoff into rivers and beaches.

Nutrients washed into catchments in wet weather attract baitfish, which draw sharks closer to shore.

Moriarty said swimmers should be wary of particularly high levels of baitfish activity off Sydney’s northern beaches.

Swimmers and surfers in NSW are being warned to be on the lookout for aggressive bull sharks. Credit: Sitthixay Ditthavong/AAP

The harbour city is still recovering from a string of shark attacks that closed beaches and claimed the life of a young boy.

Nico Antic sustained critical injuries to his legs after being bitten near a popular swimming spot at Vaucluse, in the city’s east, in January.

He died in hospital almost a week later, making it the first fatal shark attack inside Sydney Harbour in more than 60 years.

Sharks are also sighted in rivers after heavy rain.

In January, a private school cancelled rowing training on the Parramatta River after a shark was seen bursting through the water.

The NSW government has spent an extra $6.7 million on shark mitigation measures following the January attacks, including increased drone surveillance and tagging.

Swimmers and surfers should avoid entering unclear waters and should not swim at dawn and dusk.

Groups of baitfish, diving birds or dolphins are warning signs sharks might be in the water.

The SharkSmart app provides up-to-date information about shark sightings and how to reduce the risk of an attack.