Have you ever been paid more than your usual wage and not known why? Did you keep those extra funds and say nothing to your employer? A lesser known fact is that if you know you have been overpaid, there has been no explanation, you say nothing and keep that extra money, this is theft.
The Crimes Act 1961 (Section 219) states that theft or stealing is the act of, dishonestly and without claim of right, using or dealing with any property with intent to deprive any owner permanently of that property or of any interest in that property after obtaining possession of, or control over, the property in whatever manner. The property in this case, is the extra wages. The theft is intentionally, and knowingly, keeping the extra wage.
From the perspective of an employer, if you have overpaid an employee and it was a human error, you can arrange with your employee to enter into an agreement to have those funds repaid whether in full or overtime. However, if your employee does not agree to enter into the agreement you will need to apply for a Court order and prove that the overpayment was an error. You cannot recover these overpaid wages if the overpayment was deliberate or if your employee had no knowledge of the error and spent that money in good faith.