A 16-year-old girl has died after being attacked by a shark as she swam in a river in Western Australia.

The girl was bitten by an unknown species of shark in the Swan River in the Perth suburb of North Fremantle, a state government statement said.

She was pulled from the water but pronounced dead at the scene after efforts to revive her failed.

"It's very early on, what we're being advised is that she was with friends on the river," Paul Robinson, police acting inspector for the Fremantle district told a news conference.

"They were on jet skis. Possible a pod of dolphins were being seen nearby and the young female jumped in to swim nearby the dolphins."

Describing it as a "very, very traumatic incident", Mr Robinson said the family of the girl, who was from Perth, were "absolutely devastated by the news".

Fisheries experts had advised that it was unusual for sharks to be found in that part of the river, he said.

The state government warned people to take "additional caution" in the Swan River and to abide by any beach closures.

The last fatal attack in an Australian river was recorded in 1960 when a bull shark measuring an estimated 3.3m killed a snorkeller at Roseville Bridge in Sydney, according to a database run by Taronga Conservation Society.