You will only accrue interest if your Zip Money purchase has an owing balance after the interest free period (minimum 3 month) expires on that purchase.
After the interest free period ends, the owing balance of your purchase will begin to accrue interest at a rate of 25.9% per annum. The interest will be charged to your account at the end of each month and will be clearly displayed on your statement so you can keep track of how much interest has been applied.