If your employee's tax code is M SL it means that they have a student loan and each pay a percentage of their gross taxable earnings will be used to pay their loan. Part of their earnings are exempt from the calculation, this is known as the Student Loan threshold.
If you have received a notification from IRD to deduct an extra amount for Student Loan Repayments or Change student loan deduction rate for one of your employees, you are required to do this by law.
Check the details of the payday and pay period on the notice, if it is incorrect you will need to contact IRD.
The date you must start deducting is stated on the notice.
iPayroll reports this to Inland Revenue with your Payday filing.