go to the internet and decide which prefecture yu want to go to and look for high schools there and contact them directly.
Tell them you want to work as a `English Conversation Teacher`, or `Teaching assistant!`
As long as you mention that you studies all your life in English and was colonised by australia and have adopted their education system and use enlish in elementery/kindergarden up to university/job/ etc than most likely you will be hired!!
There are many young people from Canada and USA who come to Japan for 1 or 2 years to teach or assist in High Schools in Japan.
They pay good money too!! I think they also pay for ticket as well to come to Japan if you are hired. on contract basis.
Find out more about the high schools from the Japs Embassy as well.
Getting a vias to come to Japan is easy as long as you have return ticket and a guarantee letter from your host eg from the principls of the high school you will be given a visa. not like aussie.
Its also an opportunity for English teachers from PNG to experience Japanese life-style.