A 44-year-old man is in a serious condition following a shark attack at a beach south of Newcastle.

It's understood he was on his surfboard at Redhead Beach when he was attacked around 4:40pm this afternoon.

He was mauled on the upper thigh and torso by a bull shark which also took a bit out of his surfboard.

The man, suffering extreme blood loss, was able to paddle his board to shore where he was assisted by fellow surfers and treated by paramedics before being rushed to hospital.

Close to 100 people were swimming between the flags at the time of the attack and were ordered from the water after the injured surfer was helped to safety, Fairfax reports.

A witness says it was another ten minutes before before a shark alarm was sounded.

He told Fairfax his son saw the shark and the shark involved in the attack was a bull shark.

He saw the injured surfer paddling towards the shore before being assisted from the water missing "a big chunk out of his thigh".

"His skin was really grey, he obviously lost quite a lot of blood," the man was quoted as saying.

He said the beach was "bizarrely calm" following the attack.

"It was kind of like Jaws, you know, the scene at the start... where everybody's pulled out of the water and it's a hot day and the water's inviting, and no one's in it."

Redhead Beach south of Newcastle. Photo: Seven Network