Manly Beach and Freshwater Beach on Sydney's Northern Beaches were closed this morning after reported shark activity just days after a fatal attack nearby.

Northern Beaches Council announced the closure of the two popular beaches, saying the NSW Government Shark Smart app had reported a sighting of a "tagged shark in the vicinity" of the beaches.

This marks the fourth beach to be closed in the area, with Dee Why Beach and Long Reef Beach closed over the weekend after a fatal shark attack.

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On Saturday, 57-year-old surfer Mercury Psillakis was attacked and killed by a large shark off Long Reef while surfing.

Psillakis was taken by a shark described by witnesses as being aro

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