Different countries will have different laws about this type of situation. I don't know what Australian law says about this.
For example, in the UK it is illegal for you to prevent the supplier from taking back an item that was sent incorrectly, but you don't have to send it back yourself, and you certainly don't have to spend your own money to send it back.
But while the package is "in your possession", you have a "duty of care " to keep it in good condition, and take reasonable precautions against someone stealing it, etc - i.e. you have to look after it as if it was your own property.
For example you can't just put it outside your house and leave it in the rain for a month, until the supplier sends a courier to collect it!
In the UK, you should contact the supplier and agree on a way for them to collect it at a time that is convenient for you as well as for them. Usually, they would get a parcel delivery company (like DHL or FedEx) to collect it from your house, at their expense. That way, the courier's paperwork (signed by you) shows that they actually did collect it, and if anything else goes wrong after that point in time, it's not your problem!
For a relatively cheap item, the cost to the company of collecting the item may be more than it is worth, so they might agree to let you keep it - but in the UK you don't have any legal right to demand that they do that.