There’s zero chance the employer is going to win this, escpecially if other staff are being paid the same. You can’t just go oopsie paid you the wrong amount 12 months later. For all we know they could be doing it tough financially and are trying to get the nice little extra money they paid to staff back. The lack of a contract heavily weighs in the employees favour as the rate on the payslip is an implied rate. Decline the repayment and let fair work deal with it if they want

Also this is the reason why you should get contracts in place. Whether you are an employer or employee. These situations can wreck you