You can mail letters and parcels through Australia Post boxes or offices. Stamps for overseas mail cost more than Australian mail. Australia Post also provides other useful services, such as paying bills and taking passport photos (see www.auspost.com.au).
If you move house, you can have your mail redirected to your new address (go to www.auspost.com.au/parcels-mail/redirecting-your-mail.html). You may be eligible for concessions on the cost. Australia Post can also pass on your new address to other organisations.