If the advised rate is 20% and this includes the ACC levy, enter the Tax Rate as 20% minus the ACC levy. So if the ACC levy is 1.45%, enter the tax rate as 18.55%. Similarly, if the ACC levy is 1.39%, you'd enter 18.61%.
Payroll will automatically add the required ACC levy to the employee's tax to bring the total back to the required 20%.
Continue processing your employee's pay as normal.
Due to the ACC levy automatically adding to the rate entered for the Special Tax Code (STC), if the STC rate advised by the IRD is 0%, you'll need to enter the current ACC levy rate as the STC rate. For example, if the ACC levy is 1.45%, you'd enter -1.45% (0% minus 1.45% ACC levy).
An STC of 0% will result in no tax (PAYE) being deducted.
You'll be able to continue processing your employee's pay in the normal way.