The police watchdog has found two officers did not properly investigate a sexual assault complaint because the woman suffered from a mental illness.
In November 2012, a woman who suffered from mental illness complained to police saying a security guard at a hospital where she was receiving treatment had sexually assaulted her.
The Independent Police Conduct Authority (IPCA) said police took very little action in relation to the original allegation.
On 10 November 2012, the woman was admitted to the accident and emergency department after suffering an overdose on prescription drugs.
The woman was put into an observation room but she woke up to find a hospital security guard sexually assaulting her.
She fell unconscious but, when she woke up, immediately told a nurse that it had happened then ran out of the room and out to the ground where she began cutting herself
Police were called and arrived at about 1am and the woman told them she was sexually assaulted.
But the officers paid little attention to her complaint because she was well known to them.
The same guard was arrested nine months later for an unrelated sexual assault.
Police noticed the cirumstances were similar to the original complaint so they interviewed the security guard about the original allegation.
They also charged the guard for the original assault but did not tell the woman.
The IPCA said not telling the woman that the man had been arrested broke the law.
Its chair, Sir David Carruthers, said the woman had a history of mental illness and that clouded police's judgement.
He said the officer investigating the complaint did not properly interview witnesses to see whether further action was needed.
He said the officer also did not get details of the complaint from the woman.
The security guard was found guilty of sexual assault this month and is now awaiting sentencing.
Police said they had accepted the authority's findings and accepted the officers handled the situation poorly.
They said the two officers had been investigated but have not said what action, if any, has been taken against them.
They said they remained committed to treating sexual assault victims fairly and with respect.