A teenage girl has allegedly been sexually assaulted on Rottnest Island.
Police say they are investigating reports a 15-year-old girl was assaulted, between 8.30pm and 11pm on Saturday 16 September.
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7NEWS understands the incident occurred at a toilet block on the island.
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The alleged attacker was known to the girl, police say.
Anyone with information is asked to call Crime Stoppers on 1800 333 000.