She was pronounced dead after being pulled from the Swan River, in the Perth suburb of Fremantle, on Saturday.
It is believed the girl, from Perth, was riding jet skis with friends on the river when the incident happened.
Inspector Paul Robinson, from Western Australia Police, said it was possible the girl had jumped in the water to swim with dolphins seen nearby.
He described the incident as “very, very traumatic” and the family of the girl was “absolutely devastated by the news”.
People have been urged to take “additional caution” and to abide by any beach closures.
Fisheries experts say it is unusual to find sharks in that part of the river, Mr Robinson said.
This is believed to be the first fatal shark attack in the Swan River since a 13-year-old boy was killed in January 1923.
Australia typically records about 20 shark attacks each year, with most in New South Wales and Western Australia.
There were two fatal shark attacks in 2021, and seven in 2020.
Historically, dying from a shark bite is not common. In more than a century of records, Australia’s shark attack mortality rate is 0.9 – less than one person per year.