A video was posted to Instagram showing what seemed to be a large fin in Narrabeen Lagoon, which has burst its banks following heavy storms.
Shark tracking website dorsalwatch.com logged the sighting at 7.23pm yesterday.
'Great White spotted in Narrabeen Lagoon. Fin spotted from shore. Lagoon was opened up to the ocean due to storm and flood warnings,' it stated, adding the length of the shark was not specified.
Sharks have been spotted in the lagoon on rare occasions in the past.