Manly and Freshwater beaches on Sydney's northern beaches were closed on Monday following the detection offshore about 10.50am.
The shark had last been detected near Soldiers Beach on the Central Coast in June, having been tagged in 2023 at Coffs Harbour.
Dee Why and Long Reef Beach have not reopened following a fatal attack on Saturday morning which killed local surfer Mercury Psillakis.
The 57-year-old had been alerting other surfers to the shark before he was attacked, Dee Why local and former pro surfer Toby Martin said.
"Unfortunately, he was the last one furthest out to sea and ultimately paid with his life," he told Nine's Today program on Monday.
"But it's important to recognise this is just a super rare incident."
The Long Reef Boardriders Association held a memorial on Sunday evening for Mr Psillakis, who was "loved by everyone".
"Merc and (twin brother) Mike have spent more time in the ocean at Longy than any people on the planet," the club said on social media.
Premier Chris Minns called Mr Psillakis an "absolute hero" for trying to warn others about the shark.
"It's a terrible tragedy and I need to work out what the next steps are," he told reporters on Monday.
The fatal attack, the first recorded on the northern beaches in nine decades, has led to a pause on a trial removal of three of the 51 shark nets in NSW.
The nets, which only stretch 150m, are installed at beaches between Newcastle and Wollongong, including Dee Why at the start of September and removed at the end of March.
Long Reef has no net, but smart drumlines are used to monitor for shark activity and were in the water when the shark attacked.
Nearby Dee Why Beach has a shark net approximately 500m out from its surf lifesaving club.
Animals could swim around and under the nets and they offered little protection, Macquarie University natural sciences professor Culum Brown said.
"You cannot tell the difference between unprovoked attacks on netted beaches versus other beaches," he told the ABC on Monday.
"If they do have any effect, it's largely psychological rather than providing any kind of protection."
A trial removal of three nets in the Central Coast, Northern Beaches and Waverley local government areas has been paused following the attack.
Central Coast councillor Jared Wright said the council recently reaffirmed "unanimous support" for participating in the trial.
"We want to be using a mix of more modern and innovative solutions," he said in a statement on Monday.
Amid the debate, Mr Martin urged people to get back into the ocean.
"It's a beautiful place to be, the ocean ... it's a safe place."
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