Drivers most frequently test positive for the use of alcohol and/or cannabis (according to Penning Et Al). These two drugs affect driving ability in a dose-dependent manner especially when used in combination. However, the same study suggested that drivers on cocaine, ecstasy or amphetamine may not necessarily show impairment on basic driving skills, but will often overestimate their driving skills.  In combination with impaired decision making, this increases risk taking during driving.  

In a fact sheet prepared by the University of New South Wales there is a summary of Australian roadside drug testing procedures, which shows that all states and territories test for cannabis, meth/amphetamine and ecstasy. It also shows that the largest fine for driving under the influence is in Victoria, where a 3rd offence carries a maximum fine of $29,740.  

Check out the fact sheet: 

Research: Penning Et Al, Drugs of Abuse, Driving and Traffic Safety, Current Drug Abuse Reviews, 2010