Specifics of the idiom "四海一家"
As Fivesheep pointed out, “四海” means "the whole world". The word "四海一家" means "the whole world is one family". It is used to expressed that "we are family. we shall respect each other. we shall unite", something like this, to convey the politeness from the overseas Chinese people to the local people(in your case, the Australian people).

I believe this word comes from the book of 《论语》. It records a speech from a Confucius's disciple "子夏": "四海之内，皆兄弟也。",which means "[if you are a gentleman and you are polite to other people],all the people in the world are your brothers".