Paramedics were called to Shelly Beach on NSW's mid-north coast about 9.30am on Saturday, after the woman was attacked while surfing.

The 35-year-old was rushed to Port Macquarie Base Hospital with serious leg injuries, but has since been flown to Newcastle where she will undergo surgery.

Police said the woman and a man were surfing when she was bitten on the right calf and the back of her thigh.

Her companion was forced to punch the 2-3 metre juvenile white shark until it let go.

"The shark wouldn't release her and so a nearby surfer paddled over and essentially jumped on the shark and started hitting it to make it release," Surf Life Saving NSW chief executive Steven Pearce told AAP.

A surfer himself, Mr Pearce says it was a "tremendous act of bravery".

"We've had some really serious and tragic shark encounters over the past couple of months along the NSW coastline so to paddle out of your own safety zone, in to an area where you know there is a large shark, I think is amazing."

Mr Pearce is urging swimmers and surfers to be "shark smart" as summer approaches, but says the number of daylight attacks in recent times concerns him.

"As we've seen this morning, there are occasions where people can be shark smart and they think they're doing all the right things, but unfortunately, they're just in that wrong place at the wrong time."

Three paramedic crews and a specialist medical team in the Westpac Helicopter responded to the incident, which a NSW Ambulance spokesman said is the third serious shark attack on the north coast in the past few months.

Beaches in Port Macquarie have been closed for 24 hours as authorities work to track the shark.

Earlier reports had said the woman was 20.

There have been five fatal shark attacks in Australian waters in 2020.

One of them was in WA in January, when experienced diver Gary Johnson, 57, was taken near Cull Island, close to West Beach in Esperance.

In July, a 10-year-old boy suffered shock and cuts when a shark ripped him from a fishing boat about five kilometres offshore from Stanley in northwest Tasmania.

Australian Associated Press