Sydney swimmers are urged to be wary of sharks after the city was hit with a deluge of rain that totalled 100mm in just hours in some spots.

Dozens of flood rescues were carried out in the past 24 hours after a massive afternoon and overnight downpour, with more on the way.

Homes were evacuated as the rain inundated streets in Fairfield East and more than 490 calls were made to the SES.

Out of those, 42 led to swiftwater rescues.

Dozens of rescues were carried out after heavy rain triggered flash flooding in Sydney.© 9News

Cars were also flooded in shopping centre carparks while roads were cut throughout the metropolitan area.

Now, NSW SharkSmart has issued a warning to beachgoers – and river swimmers – about the increased potential for shark attacks in the wake of the heavy rain.

“Heavy rain can change beach and waterway conditions quickly,” it said in a post on Instagram.

“Freshwater and nutrient-rich run‑off can reduce visibility, affecting water quality and attract baitfish – sometimes increasing shark activity.”

People are urged to avoid swimming in murky or dirty waters, take extra care within 1km of river mouths and harbours, and to be cautious around baitfish, diving birds, and dolphins.

There was a rash of shark attacks after a recent series of storms along the NSW coast, one of which resulted in a young boy’s death after he was mauled at Vaucluse.

Experts at the time warned storm activity provided cover for bull sharks and lured them closer to shore and even upriver.​ ​

The rain should ease throughout the morning, with the Bureau of Meteorology cancelling a severe warning for Greater Sydney.

But flood warnings remain in place for much of outback NSW, Queensland, South Australia, and the Northern territroy.

Waters rose high rapidly.© 9News

A near-stationary tropical low system hanging over Central Australia continues to pour rain into all four states, with the heavy downpour slowly moves south.

Adelaide is next in line for a soaking, with more than 100mm forecast to fall on the city in coming hours.

Some events in the city have already been cancelled, with flash flooding expected across the metropolitan area.

Some streets were evacuated.© 9News

Rain also continues to bucket down in dust-dry outback Queensland, with the tiny town of Birdsville, 10km from the South Australian, copping an extremely rare drenching.

Weatherzone reported that in the 24 hours to 9am yesterday, the town received 93.2mm, more than half of its annual average of 161.4mm.

It was also Queensland’s second-highest 24-hour rainfall total behind Fall Creek in the decidedly more tropical Cape York Peninsula.