There is no legislation which describes the way investigations are to be conducted, but if an employer relies on an investigation approach to justify dismissal or other disciplinary action, a court will take a dim view if it appears the investigation was not conducted in a reasonable manner and failed to follow basic principles of procedural fairness. So for example if an employer sacked a worker on the basis of sexual harassment, but had failed to either verify any of the allegations made by a victim, and failed to give the accused worker a reasonable opportunity to give their side of the story, a court would likely find the dismissal was unfair and award the worker damages.
What constitutes reasonable in any particular circumstance will vary, but it is true to say that courts expect larger organisations to address matters in a formal and procedurally reasonable way than they would say an employee with only one or two staff.
If you need to look up legal cases I would recommend Austlii which is a publically available legal database with most recent significant court and tribunal cases, and which you can search by terms:
They include a database of FWA cases too, so you likely will find something on point if you try using a few different terms.
Good luck and PLEASE RATE MY ANSWER.