What are we comparing mate? Your 15,000 murders per year to our 240? Using your population figures, the USA has 13.4 times the population of Australia, so if I multiply 240 by 13.4, that'd take our annual murder rate up to 3240 if we had the same population, around a quarter of the US' annual murder rate?
Please don't consider this as USA bashing, it's not, I'm only writing this in response to your post, and you're right, we have the best border that you could ask for, it's just a pity that our government seems to think that importing 300,000 immigrants annually (mostly from China and the Middle East) is a good idea. We've all got our problems.......