The official language in Australia is English. If you wanted to say goodnight you could do so in English. Some common ways to say goodnight would be, goodbye, bye, see you later, tata, or later. Though people in Australia speak English they also have a lot of slang terms that they use when speaking to each other. You may hear some in Australia saying cheerio or hooroo rather than goodbye. G'day is a slang term used for hello.