Poste Restante is an international postal convention, operating routinely in Australia , as in most countries. Many travellers in Australia rely on it, but many others, particularly non-English speakers, seem not to be aware of it here.
Australia Post will reliably hold mail marked "poste restante" for at least a month at any Post Office or agency anywhere in Australia - longer if the sender indicates a different period. There may be a fee for parcels held longer than one month.
Any postal product (letter, parcel etc) can be forwarded to, and received from, the Poste Restante.
At the end of that time, if the item is not claimed, it will be returned to the sender, generally free of charge.
Your mail should be addressed :
[ Your Given name + FAMILY NAME in large print]
Poste Restante
[TOWN] - [state] Australia - [postcode, if possible]
To collect your mail, simply advise anyone who wants to write to you of your planned route and approximate date of arrival at each place. All towns in Australia, even quite small ones, have at least an agency of the Post Office, though you may need to check with a major post office before you leave, if your map indicates no other services at all at the destination (eg no petrol or hotel).
You will need to present some form of identification to collect your mail.
At the time of writing, there is no extra fee for this service, apart from the cost of the postage paid by the sender. As unclaimed mail will be returned, eventually, it is also reasonably safe even if you happen not to collect it.
This service is useful even to send a letter or parcel to yourself at some point on your future journey (see above for parcel information).
One possible difficulty is that some Post Offices offer a 5 day (only) service, that is , they may be closed on weekends. So you may need to plan your arrival to coincide with opening times.
G.P.O.
There is a magnificent G.P.O. building in Martin Place between George Street and Pitt Street. Note, though, that Poste Restante is not here. It is in a separate office back from the street, but clearly marked, a little further along George Street towards The Rocks.