Regardless of where you lodge your complaint (i.e. with the police or with IBAC), in almost all circumstances, it will be a police officer who investigates your complaint. A complaint made directly to the police will be handled according to how serious the PSC considers your allegations to be.
A complaint of serious misconduct (e.g. the alleged conduct amounts to a criminal offence that is punishable by imprisonment, or is disgraceful, or is likely to damage the reputation of the police force) is required to be investigated formally.
Police have the discretion to not investigate less serious complaints, but to deal with them as “customer service issues”. These complaints (e.g. allegations of rudeness) are usually handled quickly and informally at a local or district police management level. Police also have the discretion to dismiss trivial complaints. If you think the PSC has wrongly characterised your complaint as trivial and wrongly dismissed your complaint, you can contact IBAC.
Police investigations usually involve at least one interview between the police officer who is investigating the complaint and the person making the complaint. You do not have to give an interview, but it usually helps your complaint if you do. The length of the interview depends on the seriousness of the complaint, how clearly the complainant explains the issue they are complaining about, and the complexity of the scenario into which an investigation is being undertaken. Subsequently, police officers will sometimes interview witnesses and make various inquiries. Sometimes, legal and scientific advice is obtained.
Once an investigation is completed (this typically takes several months), the results of the investigation are referred to the officer in charge of the PSC, who decides whether or not the investigation has substantiated the complaint.
The outcome of the investigation may also be reported to IBAC.