Sorrento Beach, one of the busiest stretches of sand in Perth's northern suburbs, was closed this morning after a suspected shark attack that has left one swimmer missing.

St John WA received the first reports at 10.15am and sent three crews to the beach, including one on priority. Within the hour, water police, a police helicopter and a police boat were working the water off the groyne, and a crowd had gathered on the footpath above the beach watching the search unfold.

Surf Life Saving WA confirmed the response in a post on Friday morning, saying DPIRD Fisheries officers "are currently investigating a possible shark incident at Sorrento Beach, Joondalup". The Department of Primary Industries and Regional Development logged the incident on the state's SharkSmart WA alert list the same morning.

What witnesses saw

Jacqui Rapaic was walking along the beach with a friend when it happened. She told the ABC her friend spotted it first.

"My friend said, 'Jack, I think there's been a shark attack' and I looked out to the ocean and I could see a pool of blood." — Jacqui Rapaic, witness, speaking to the ABC

Ms Rapaic said she believed two men had been swimming, and that only one of them made it back to shore. "I hugged onto my friend… [we] felt rather traumatised at that point," she told the ABC. "It happened so quickly."

Another beachgoer, Russell James, was there when the shark alarm sounded. He described a helicopter circling overhead and a police boat coming in, with police and ambulances gathering near the water's edge by the groyne, where a couple of surfboards or paddle boards were lying on the sand.

Before the incident, a member of the public had reported seeing a shark roughly 2.5 metres long off the beach.

What is confirmed, and what isn't

As of early Friday afternoon, the position is this: a swimmer is missing, the beach is closed, and DPIRD Fisheries officers are investigating. Several outlets — including The West Australian and Nine — are reporting the attack as fatal, and Nine reported the swimmer is feared dead. Police had not issued a formal confirmation of a death at the time of writing, and no one has been publicly identified.

St John WA told 7NEWS its three crews had since been stood down. Police were continuing to patrol the shoreline, and swimmers have been told to stay out of the water and take caution along the surrounding coast.

The second WA shark attack this week

This is the second reported shark attack in Western Australia in five days.

On Monday morning, 56-year-old firefighter Mel Ismail was bitten by a shark while surfing at Glenfield Beach in Geraldton, about 400 kilometres north of Perth. Fellow surfer Shannan Dunn, who had photographed the swell minutes earlier with Mr Ismail visible in the distance, paddled out to help him. "His board was about 5 metres away, and he had his arm up," Mr Dunn told the ABC. "Then it only took a couple of seconds and I saw the blood all around him." Mr Ismail was taken to hospital; Mr Dunn returned to the beach for the first time this week and says he plans to surf again soon.

A shark warning had already been in place for Perth metropolitan waters since 11 September.

Why it matters for Perth

Sorrento sits about 20 kilometres north of the CBD, just south of Hillarys Boat Harbour and its strip of restaurants and cafes. It is a family beach, a swimming beach and a dog-walking beach, and on a warm spring morning it is busy — which is exactly why today's scenes have carried so far so fast.

The timing is pointed. Surf Life Saving WA only began its weekend helicopter shark patrols a week ago, gearing up for the summer season. That season effectively starts now, and it starts with a beach closure and a search.

Anyone who sees a shark is asked to report it to Water Police on 9442 8600. Current alerts and warnings are published at sharksmart.com.au, and the free SharkSmart WA app pushes notifications for beaches you follow.

This is a developing story and we will update it as WA Police and DPIRD release more.

Photo: 247Channel via Wikimedia Commons (CC BY-SA 4.0). This is a file photo of Sorrento Beach, not an image from today's incident.