How to apply for a Japanese visa in Australia is described in detail here.

It depends on your identity. In what capacity do you live in Australia? If there is PR, it will be much easier. If you don't have PR, it will be relatively difficult. The specific procedure is: 1, and you must have a visa guarantee to return to Australia, and you can fully explain that you will not stay when you come back from Japan (such as house certificate, school certificate, Australian visa, etc.). ) .2. Your friend's parents will send you an identity certificate to ensure your behavior in Japan, explain to the embassy why you want to go to Japan, and ensure that you won't stay, no. 3. Explain who is responsible for the travel expenses (air tickets). 4. Detailed visit schedule, where to go, what plane to take, where to stay, etc. 5. No guarantee is required.

P.s.' s question about the invitation letter. Your friend's parents wrote to the consul of the Japanese Consulate in Australia in Japanese. The specific consulates vary according to your consular district, such as the Japanese Consulate in Melbourne, the Consulate in Sydney and the Consulate in Perth. The main content is your detailed schedule in Japan, to ensure that you can leave Japan due, your relationship with the inviter (friends or others), to ensure that you will not break the law, and to ensure your behavior in Japan (mainly that they are willing to guarantee your responsibility after breaking the law). The school certificate of high school students is also ok. Mainly your residence certificate (house contract, which ensures that you can stay after you come back from Japan) and bank deposit certificate.