Australian Data from the Queensland Criminal Justice Dept in 1993 shows that, in only 3 years after decriminalisation of cannabis in 1987 there was a 21 to 3-fold increase in its use in South Australia compared to other states.  Adelaide is reckoned the drug capital of Australia with use of LSD, amphetamines and ecstasy up to 3 times greater than Sydney.

Comparing the use of cannabis in the Australian Capital Territory  (ACT) with New South Wales after the ACT trivialised penalties for use to a $100 fine, as against $2000 in NSW. Adolescent male use was found to be 55% higher and female adolescent use 83% higher than in NSW.

References
1. Cannabis and the Law in Queensland Advisory Committee on Illicit Drugs. July 1993
2. Attitude, ABC TV May 1993
3. A.C.T. Board of Health Schools Student Survey. 1991: Years 7-11.