Seems to apply to both new and old users. Terms and conditions don't say all that much:
Transaction must be $100 - $499 in order to receive $50 cash back; or $500+ in order to receive $100 cash back. Transaction must be made between 16/09/2020 and >30/09/2020 11:59PM. Limit of one cash back per customer in promotional period, which will be applied to first eligible purchase. Based on purchase amount, $50 cash back or >$100 cash back will be removed from the final scheduled payment.
Can be combined with other coupons such as CottonOn's 20% off using: SPRING20
Can someone give a tldr of how this works, more like afterpay or more like zippay