The general manager of a council that has made a substituted rate in respect of a financial year is to calculate, in relation to an area of land that is rateable land to which the substituted rate is to apply, the difference between –

(a) the amount of all rates for the financial year payable by the ratepayer in respect of the area of land, as contained in a rates notice sent to the ratepayer in accordance with section 122 ; and

(b) the amount of all rates, for the financial year, that would have been payable by the ratepayer in respect of the area of land if the rates notice had contained, instead of the amount calculated in accordance with the previous rate, an amount calculated in accordance with the substituted rate.