The federal government is being urged to lead a national approach to reduce shark encounters in Australian waters after a spate of attacks has left victims fighting for their lives.
Among the four victims across three days is a 12-year-old boy who sustained critical leg injuries and remains in hospital.
Shark mitigation measures such as nets and drum-lines were simply not working, Greens senator Peter Whish-Wilson said.
He argued they typified "a lazy and ineffective approach" that needed updating based on the science.
"The federal government needs to show leadership in striking the right balance between reducing the risk of shark encounters and conserving nature," Senator Whish-Wilson said.
"Palming off responsibility for shark bite mitigation to the states is a cop-out given many sharks and other marine creatures killed in lethal shark control programs are federally protected species."
Leo Guida of the Australian Marine Conservation Society said a "nationally consistent approach" was worth exploring.
But he urged the states to better invest in technologies that work and which could be scaled across Australia.
He rubbished the use of shark nets as a "placebo" that gives a false sense of security to swimmers, adding that two of the recent four shark bites occurred at netted beaches.
"It's like having a flyscreen door with a mesh only as wide as your thumb and hoping to keep flies out - it's not going to work," Dr Guida told AAP.
"Sharks can swim over, under and around them."
Twenty beaches were closed on Tuesday, including along the 30km coastline marking Sydney's northern beaches, as well as at surfing hotspot Crescent Head, near the site of Tuesday's incident.
The shark scientist said NSW having the world's largest shark tagging program was a model to follow and other "evidence-based methods" such as drone surveillance were where taxpayer dollars needed to be spent.
Australia's white shark population across its coastline is estimated to be at fewer than 500 breeding adults, according to latest research.
A 39-year-old man was taken to hospital after being bitten in the chest at Point Plomer, north of Port Macquarie on the NSW mid-north coast, on Tuesday morning.
Two people - including the 12-year-old boy - have been left with serious injuries after shark bites in two Sydney incidents, while a second boy's surfboard had chunks bitten out of it.
NSW Premier Chris Minns said he didn't have a "silver bullet" conceding "warning system and communication needs to be beefed up, particularly during stormy weather".
More than 30 SMART drumlines have been deployed along Sydney beaches, while shark listening stations and aerial surveillance have been ramped up.
