Australia Post will hold your parcel for 10 business days, giving you plenty of time to collect it. If you can't pick it up within that time, they'll return it to the sender.
When you go to the Post Office, just bring your ID. You'll need to show that you're either the person it's addressed to (the 'addressee') or someone authorized to collect it on their behalf.
You can visit Australia Post FAQ page for the list of IDs that you can bring.