Employee withholding pay

Hi All,

Not really sure if this is the right sub to post in but thought it was a good place to start. I am Australian and have recently given notice. Annual salary that is payable on the 15th of each month. Shortly before I finished today one of the managers pulled me aside and said they were withholding my pay for the month and my final pay (annual leave and hours worked etc.) until I payed back the bonuses and monies id been paid/given over the last 6 years. I have two little girls and this will all but eliminate Christmas gifts for them. I assumed the extra money id been given over the duration of my employment was simply for the amount of hard work I've put in. Can my employer legally do this? I plan on contacting the fair work ombudsmen on Monday. They are withholding $6,000+ which covers my hours worked and annual leave and hours worked and want another $8,500 on top of that. None of the bonuses or monies paid to me were ever agreed to be payable back to the business in writing and I have never signed a contract saying I would pay it back. Any advice would be greatly appreciated.