A diver has died after being bitten by a shark in Western Australia and his partner treated for shock after witnessing the horror attack.

Gary Johnson was killed by a great white shark off Cull Island, near the south coast town of Esperance, 713km southeast of Perth at 1.30pm on Sunday.

His partner Karen Milligan was reportedly in a boat just metres away.

"A man received fatal injuries after being bitten by a reported white shark," the Department of Primary Industries and Regional Development said.

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Shark victim Gary Johnson who was attacked off Cull Island in Western Australia.
Shark victim Gary Johnson who was attacked off Cull Island in Western Australia.

Police said Johnson was in the area on a boat, and diving at the moment of the attack.

"A woman who was on board the boat at the time of the incident has been returned to shore, and has been taken by St John Ambulance to hospital," police said.

She was suffering from shock.

The search for Johnson's body is still ongoing as fisheries officers, police air wing and divers conduct patrols in the area.

Glen Quinlivan, one of the first responders, told the West Australian he was setting up his boat for a day trip when he heard cries for help.

Karen Milligan and her partner Gary Johnston, who was attacked by a shark off Esperance.
Karen Milligan and her partner Gary Johnston, who was attacked by a shark off Esperance.

"We tried to find him. We tried to help her but to no avail," he said. "I really feel for her, she's obviously witnessed something you don't want to see."

A SharkSmart alert from the Surf Life Saving WA Twitter account warned of a public report of a white shark at Cull Island at 9.39am WA time on Sunday.

At 1.06pm, another alert said Esperance fisheries officers were "investigating possible shark bite incident".

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It's the second fatal shark attack in the region in less than three years.

Laeticia Brouwer, 17, died after being attacked near Wylie Bay in 2017.

She was on a family holiday and in the water with other surfers including her dad.

Sean Pollard was attacked by two great white sharks nearby in 2014, losing his left arm and right hand in the attack.