Every year, on September 1, about 50 shark nets are reinstalled at beaches up and down the NSW coast and stay there until the end of April.

Queensland has 27 nets along its coastline, but they stay in the water year round.

Both state governments say the nets are an important part of their shark control efforts.

But conservationists and academics who've studied them say they are nothing more than a feel-good gesture for a fearful public, and take a horrific toll on marine life.

Earlier this year, the NSW government revealed target shark species accounted for just 13.5 per cent of everything trapped in the state's nets last season.

Threatened species accounted for 22 per cent of the 376 animals caught, and included critically endangered grey nurse sharks, vulnerable green turtles, endangered leatherback turtles, and endangered loggerhead turtles.

Queensland has seen 10 whale entanglement incidents so far this migration season.

Protests, timed to coincide with Thursday's reintroduction of nets in NSW, were planned well before Wednesday's attack at North Avoca Beach on the Central Coast.

A 14-year-old boy was bitten on the hand at the popular beach about 7am. He's since been released from hospital after having the wound stitched.

Avoca and North Avoca beaches were closed after the attack, with authorities urging people to stay out of the water. The incident, said to involve a white shark, is being investigated.

The NSW Department of Primary Industries said baited drum lines were set at both beaches at 7.13 and 7.17am on Wednesday, as per normal practice.

Lawrence Chlebeck is a marine biologist with the Humane Society International Australia.

Mr Chlebeck expressed sympathy for the young shark victim. However, he said the drum lines clearly didn't protect him, and the nets wouldn't have helped either had they been in the water.

He points to the fact that there have now been 37 shark bites at protected beaches in NSW, and about double that number at protected beaches in Queensland.

"Bites have still occurred," he said.

"The presence or absence of nets in the water would not have made any sort of difference. That equipment does not form a barrier and sharks can easily move inside the nets, above and around as well.

"And drum lines are ineffective at reducing risk. If anything they could potentially be attracting sharks inshore. I'm not saying that's what happened on this occasion, but it's very possible."

Protests are planned on Thursday at Manly in Sydney, Byron Bay in northern NSW and on Queensland's Gold Coast.

Australian Associated Press