Lifeguards erected the signs at the lake foreshore on Monday, October 5.

Swimming near the bridges is not advised until further notice.

Low water quality meant swimming in the lake was already not advised.

NSW DPI shark biologists subsequently confirmed identification of the shark as a grey nurse shark.

A spokesperson said this is not considered to be a potentially dangerous shark species.

Saving Lake Cathie's Stewart Cooper said the shark was another chapter in the lake saga.

"It would be nice to see it removed," he said about the shark.

The lake is closed to the ocean.

Revive Lake Cathie president Danielle Maltman said they had received reports the shark was 2.5 metres to three metres in length.

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She encouraged the general public to take heed of the warning signs and not swim in the lake.

Mrs Maltman said Revive Lake Cathie was concerned about the water quality and the shark.

"There needs to be more warning signs," she said.

Lake Cathie Amateur Fishing Club secretary Terry McDermott's research shows professional fishers have caught bull sharks in the Lake Cathie-Lake Innes system over the past nine years.

"I'm aware of our [club] fishermen seeing sharks under the bridge from time to time, with us not necessarily identifying the species," he said.

"It is probably reasonably rare but it happens."

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