A man has been killed by a four-metre shark at Horseshoe Reefs off Rottnest Island.

Frantic attempts were made to save the 38-year-old, who was pulled onto a boat and taken to shore, where he was attended by paramedics on a wooden jetty.

7NEWS vision showed a dozen people were huddled around the wounded man, who was partly covered by a blue blanket.

Police were also at the scene, with a water police vessel moored nearby.

Emergency services were scrambled just before 10am. An RAC Rescue helicopter was sent to the incident.

WA Police said it was advised of a possible shark attack off Geordie Bay at 9.55am.

“A 38-year-old man was in the water at Horseshoe Reefs when he was believed to have been bitten by a shark,” a police spokesperson said.

“The man was conveyed by vessel to shore, where he was met by SJA WA paramedics. Sadly, the man was unable to be revived.

“WA Police will prepare a report for the Coroner.”

The Department of Primary Industries and Regional Development said it was alerted to the shark attack at 10.03am.

“DPIRD is working with WA Police to coordinate responses,” a spokesperson said.

“A DPIRD patrol vessel will be on the water monitoring the area.”

Horseshoe Reefs is north of Marjorie Bay, at the west of the island. DPIRD had earlier reported the incident occurred at Geordie Bay.

More to come.