Locals on Sydney's Northern Beaches have spotted what they believe to be a monster great white shark in a lagoon that broke its banks during the city's heaviest downpour in 22 years.

There are reports this morning that a great white shark has been lurking in a flooded Sydney lagoon close to homes and where, just yesterday, locals were swimming.

Footage emerged on social media during wild weather on Sunday showing what appeared to be a large dorsal fin in the swollen Narrabeen Lagoon on Sydney’s northern beaches.

The user who uploaded the footage to Instagram is an Airbnb host at Narrabeen who captioned the video: “Great white spotted in Narrabeen Lagoon”.

Lake Narrabeen has flooded surrounding streets including Pittwater road and the backyards of those who live on the lake despite this volume of water draining out to sea. #NSWFloods #SydneyStorm pic.twitter.com/aE2GkykIMc — Andrew Larsen (@LarsenDrew) February 9, 2020

The website Dorsal Watch, which lists shark sightings around the world, featured a report at 7.23pm from North Narrabeen of a “fin spotted from shore”.

“Lagoon was opened up to the ocean due to storm and flood warnings,” the report read.

Earlier in the day, locals were photographed running and jumping into the lagoon as heavy rain continued to pour down.

A shark was last spotted in Narrabeen Lagoon in January 2007. On that occasion, two men said they saw a great white swimming out of the lagoon after earlier reports from a fisherman who said it pulled a duck under water.

Stephen Burleigh told The Sydney Morning Herald what he saw.

“There were half a dozen ducks in the water,” he said.

“I saw four of them moving very quickly to the side but the shark got one and dragged it under the water, five metres under.

“I was worried if there were any children in the water. I stepped back and said, ‘I’m not fishing here any more’.

“It’s school holidays and a lot of kids come here.”

The report of a shark swimming in flooded waters on Sunday was followed by the evacuation of some residents near the lagoon.

The Bureau of Meteorology has forecast more severe weather on Monday including heavy rain, strong winds and damaging surf conditions that are likely to further impact the northern beaches.