like a lot of observations and generalisations made on here, i think the issue of ethnic tension (or not) depends entirely on where you are as to what you experience.
sure, there are "gangs" of various ethnicities, but i see gangs of various subcultures around the place too - surfers, hip hop kids or whatever. we belong to whatever we belong to, socially - but i really think that australia is one of the most culturally diverse, harmonious countries in this regard. there are only a minority of ethnicities that would really feel threatened or ostracised here, and a lot of this has to do with media or political hype (ie people of middle eastern descent) or history (such as the indigenous population). there might be parts of western sydney or whatever where the community isn't cohesive, where cultural divisions are an issue, but i think in the scheme of things, our community is pretty accepting and cooperative on the whole.
yes, every wave of immigration has been a challenge to certain parts of the anglo-australian outlook, but so far we've (speaking for myself) adapted pretty well to each of them. if we weren't bombarded with all this islamic paranoia in the decade after 911 i really things would be different in regard to how muslims, arabs etc are viewed by australians (whatever 'australians' even means nowadays) but outside of a bit of racism and a bit of ignorance, australia is a pretty ideal 'multicultural' nation. we don't have the ghettos that exist within the US, britain, france or a lot of other comparable nations, and all in all we work together pretty well.
i only want to clarify my comments because i'm very suspicious of this recent shift against the idea of multiculturalism.
my view is that we have aeroplanes, people move around the planet as it suits - and this is not a bad thing - it's something we're all going to have to deal with.
it's also misleading to confuse "multiculturalism" with "immigration" and "refugee intake" - these are three completely separate issues, but often get simplified in media debate as one thing, which does the discussion no justice IMO.
for the record, i've only ever had issues with pissed white fellas out on the town. everybody's experiences and hometowns may vary though.