I live in Sydney and it happens quite a lot, especially in some places, and at certain times - like Saturday night, when lots of people are drunk.
Are you sure it was an Aussie who hit him? There are a lot of tourists here who get pretty rowdy too.
What are the specifics - time, and suburb? There are some nasty areas in the inner city, and there are some suburbs you wouldn't want to go to during the day let alone at night.
Sydney has a lot of suburbs that are like a world unto themselves. Some Sydney people would read what I have written and think "no way, Sydney is perfectly safe" but they might be completely insulated from the worst areas. I know a lot of people who not only have never been to those areas, they have never heard of them.
Having said all that, guns and knives are illegal in Sydney. So anyone with a gun is probably out to commit a crime, not a random act of violence. (not always true, but most of the shootings are gang and crime related). Most people do not carry knives, but some do.
The downside of that is, most people don't expect to confront a weapon, so they may be more likely to start fisticuffs.
I've known quite a few get attacked for no reason, just walking down the street. Being a big scary bodybuilder is no security, it probably just makes you more of a target for a bigger initial hit.
Again, having said all that, mostly you would not have a problem.
I've been to Canada and it is much more "chill" than here. This city is very intense, fast paced with work / traffic and then people like to let off steam. People get worked up over sports events etc.. and there is plenty of booze related stuff.
But again, it is mostly a very safe place as long as you avoid the trouble spots / times, and if you don't avoid them, if you are a little bit aware.
You can walk around and completely let your guard down 100% most of the time but if you go to say Kings Cross (the redlight district) on Saturday night where tonnes of tourists are + locals + everyone is drunk as can be then you might get problems if you find yourself picked on.
There are also suburbs and hangouts that are dominated by a specific ethnic group(s) who might pick on you because you aren't.