This may be part of the problem (although no stats for the last 3 years):
"Offending rates were highest for both males and females aged 1519 years. Within this age category, most violent offending peaked around 17 years of age. However, the rate of sexual assault offending by 15 year olds (64 per 100,000) was greater than that of 17 year olds (56 per 100,000)"
This seems to fit with the age group in violent gangs and house invasions that I read about.
Such a young age group for violent crimes suggests that part of the problem may be that many violent criminals are first-time (convicted) offenders in terms of serious, violent crimes. Long sentences etc probably have limited effect on people who have not yet been convicted of a crime, and violence seems to decrease with age (those already in prison might therefore be less of the problem on average). Hence my earlier comment about value systems and young people - finding a way to stop them starting would seem more effective than forever increasing the length of punishment once convicted.