The census results are a little misleading by themselves. They indicate which languages are used for primary communication in homes, and in that sense Mandarin is not in the least bit challenging the primacy of English in Oz. But there are in fact a great many more Mandarin speakers in Australia.
And actually, I went looking for numbers on which languages are spoken in Oz and found for instance in Sydney, the second most commonly spoken language is Arabic. Mandarin is third.
http://www.smh.com.au/nsw/talk-of-the-town-youre-all-talking-my-language-20140710-zt3s9And the statistics make it look like language groups congregate. (Or at least that language clusters exist.)
http://www.smh.com.au/data-point/sydney-languagesAnd one can find quotes like this one:
Non-English speakers outnumber English speakers in one in five [Sydney] suburbs, cementing the city's status as one of the world's most linguistically diverse cities.But grouping languages as English and not-English really doesn't tell us much. I'm pretty sure that English compared to any one other language will show English as massively dominant, but I don't have ready numbers available at the moment
Anyway, the reason I've been going into this a bit more is there's one of those globalisation, naturalisation, humanisation processes at work, and it stumbles on the same point that similar equalisation trends do: when the dominant population is unwilling to give up the fruits of domination. You can see it in modern debates about feminism. Women and girls talk about not being feminists because feminism is toxic now, and men come up with men's rights groups, and people talk about waves and progress and backsliding and so on, and it's all bullshit because entirely and only the one stumbling block is men. men are obliged to stop asserting their "natural" rights, and they don't want to. They're supposed to recognise when they are asserting unnatural authority, and to stop doing it. But they don't because it means not just ceding power, it means extra work for them - the work of not stepping on other's opportunity. I'm not expressing this well, it's just something I've observed in such debates, that men want to keep the privileges of former dominance mostly because they have to do less communicative work as a result: the dominant population sits in power while the subordinate population orbits around them.
Long story short, it works for language too. Where English is dominant, the English speakers can tell all other language groups, "This is Australia, speak English" and that mostly benefits only the English speakers.
It's quite remarkable the parallels too. I recall people used to say how convenient and natural it was for men to work and women to mind the home. And look, how convenient and natural it is for any bugger coming to Australia being allowed or not allowed based on English ability.