Detective Sergeant Mark Keane, who supervised the case, was criticised in the IPCA report for "poor oversight" and Detective Dale Forman, who interviewed Ms Foley, was not a trained specialist adult sexual assault investigator as was required under police policy.
Ms Foley was never formally interviewed by such a specialist, the IPCA noted.
A day after speaking to Ms Foley, Mr Forman told the nurse the complaint would "go no further". He also failed to formally interview the nurse or potential witnesses to the alleged incident.
Giving evidence at the inquest today, Mr Keane said he attended Ms Foley's death. She had a long history of involvement with the mental health system, had a history of suicide attempts and was considered by health professionals to be a long-term suicide risk.
Ms Foley was admitted to Hillmorton Hospital under the Mental Health Act in March 2009 and transferred to the Seager Clinic in April. In October and November that year she had periods of extended leave with her parents, Ebony and Peter Foley, before being discharged to live with them.
Detective Sergeant Keane said that after Ms Foley's parents contacted police he took over investigating the sexual abuse complaints in April 2010.
After reviewing initial actions and scene examinations done by Mr Forman, interviews with staff, and reviewing Ms Foley's medical file and the nurse's employment file he concluded her complaint could not be proceeded with because of "insufficient evidence".
Mr Keane said after the complaint to the IPCA he was involved in responding to the authority's inquiries.
The inquest was held on the day Ms Foley would have turned 36.