I found this extract on the Office of Fair Trading web site. I hope it helps.
Interest and discounts on levies
An unpaid levy attracts interest at the rate of 10% simple interest a year if not paid within one month of the due date. The owners corporation cannot increase or decrease the interest, but it can make a special resolution to charge no interest. Unpaid levies, including interest, can be recovered by the owners corporation as a debt in court.
An owners corporation may make a special resolution to give a 10% discount where a levy is paid before the day it is due. Payment made on the day it is due does not attract the discount.
A simple interest calculator which you can use to input your own numbers to determine whether your interest has been calculated correctly is linked below. Scroll down the linked page to find the calculator.