See:

http://goo.gl/QpaVq

and

http://www.cycle-helmets.com/clarke-helmet-policy.pdf

Summary:
Australian bicycle hemet laws have been in place for decades now and have achieved no positive outcome in terms of bicycle safety and accident rates and ushered in a significant drop in the popularity of cycling. More cycling equates with a healthier society.

Details in abundance in the two links above.