Well for starters, I believe many gun crimes are committed with illegally owned guns from what I recall. 2nd, America has things that Australia does not. Australia has no land neighbors, and doesn't have to deal with illegal immigrants like the US does. Some of whom add to our crime rates.
Then we also have cities and states with strict anti-gun laws, where only criminals can get their hands on guns-as they don't obey the law anyway and can just get guns illegally, and when only bad folks have guns, they have free range to do what they want with little fear. If I have a gun, and you don't, why would I be afraid to rob you? Likewise, if you had a gun and I didn't, I'd think twice about robbing you.
So I would say that yes, guns do deter crime, but they also help commit crimes. Just depends on who has them. Put them in the hands of good brave folks who only want to defend their home and family, and you have no problems. Only let them in the hands of folks who grab them illegally and with bad intentions and crime goes up.
So that's why I think we have more murders.