Answer:
No, you cannot provide your own refunds and returns policy it if does not meet the requirements of the Australian Consumer Law (ACL), Schedule 2 of the Competition and Consumer Act 2010 (Cth).
When you sell to consumers (i.e. to the public and transactions under $40,000) then the ACL will apply. You must meet the mandatory Consumer Guarantees including rights to a refund or repair if there is a fault. You cannot get out of the mandatory Consumer Guarantees. You can only add clarification, and limit your liability for risks outside of the ACL. Therefore if you have specific additions or guidelines for your returns these should be set out in your Sales Terms and Conditions.