You must purchase your Japan Rail Pass in Australia before commencing travel to Japan. The other only solution would be to get your travel agent to mail it to your hotel in Japan by Courier or Express post, however this is not recommended.
Japan Rail Passes are available online can can be purchased at Japan Rail Pass Now. You can order the pass online and they will send you the JR Pass "Exchange Order" to your designated shipping address.
Treat the "Exchange Order" as your physical ticket, because once you arrive in Japan you will have to exchange this for your actual Japan Rail Pass which you will use on the Japan Rail Network.
How To Exchange:
You will need to take your "Exchange Order" with you to Japan, so make sure it's kept secure with your passport and other belongings. The validity of the "Exchange Order" is valid for three months from the date of issue.
When you arrive in Japan you will need to enter the country as a "Temporary Visitor" to obtain your "Temporary Visitor stamp". The immigration officials will put it in your passport. If you going to Japan to work or study then you will not obtain this stamp, and you won't be able to obtain your Japan Rail Pass.
The next step is to find a Japan Rail Pass Exchange office, you can find locations here: "Exchange Order" . This is where you will turn your "Exchange Order" in to your actual Japan Rail Pass. Please note, you will need to take your passport with your "Temporary Visitor Stamp" to get it, failure to do so will result in not getting your Japan Rail Pass. But don't worry, as long as you enter the country as a tourist you will be fine.
You will then need to fill in a short application form which can be obtain at the official site.
Hooray! You are now ready to start using your Japan Rail Pass. You now have 30 days in which you can activate your Japan Rail Pass, when you activate your Japan Rail Pass it will start your unlimited travel on the JR network for 7/14/21 days.