The Australian Bureau of Statistics has made an estimate of the size of the illegal drug economy in Australia. They report:

Results for the 2010 year suggest Gross Value Added (GVA), Household Final Consumption Expenditure (HFCE) and Imports for the illegal drug economy is 0.5%, 1.0% and 0.4% of total Australian GDP, HFCE and Imports respectively. This paper has applied the OECD recommended methodology to estimate the value of the illegal drug economy in Australia.

Please note that these are not to be regarded as official statistics.

The report can be found by clicking here.