A Western Australian beach south of Mandurah has been closed after a teenage boy was bitten by a two-metre shark.

Cameron Pearman, 13, said he was surfing at Pyramids Beach, about 85 kilometres south of Perth at 6:00am on Saturday when he was bitten on his lower leg.

"I was just sitting out the back waiting for a wave, and something hit me from behind and knocked me off my board and hit my leg," he said.

"I only saw its head for a split second, because I didn't really know what was going on, so I just saw it for a split second and then it swam off."

He was just south of the Dawesville Cut, a popular surfing break.

Cameron said the adrenaline rush meant he did not feel much pain, but after the shock wore off he started to feel it.

He was helped ashore by his cousin Sam and life savers bandaged the puncture wounds. He then hopped on his bike and rode two kilometres home.

His parents then insisted that he go to hospital.

"I had some brekky as well before I headed to the hospital," Cameron said.

"Put some Betadine on it, had some food, went to the hospital."

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Cameron's father drove him to Peel Health Campus where he presented at the emergency department.

He said he did not need stitches, but had precautionary X-rays to make sure no teeth had broken off in his leg.

Cameron was discharged but was later taken to Princess Margaret Hospital in Perth, where he spent the night after undergoing minor surgery.

A surfer since he was three years old, Cameron said he was planning to be back in the water as soon as he could.

"I'm planning to go for one [a surf] tomorrow morning," he said.

"Probably not the same spot for a while, but somewhere else."

But he said he would be a little warier of the water now.

Port Bouvard beach, about 85km south of Perth, has been closed after the reported attack. ( ABC News: Katrin Long )

"I've never really thought about them because I've never encountered one, but I think a bit differently about them now."

A Surf Life Saving WA (SLSWA) spokesman said it was a lucky escape as the bite was not too deep.

The species of shark is not known.

In a tweet, SLSWA said a medium-sized shark of unknown species was spotted at 7:45am near the Port Bouvard Bridge.

Authorities closed the beach for 24 hours and it was expected to be reopened on Sunday morning.

In March this year, diver Michael McGregor disappeared while several kilometres off the Dawesville Cut.

His body was recovered days later with evidence of shark bites.