Roadside Saliva Testing: Defence of “honest and reasonable mistake of fact”

It is an offence to drive with THC present in your saliva. Police can stop a driver for no particular reason and require them to provide a saliva sample. If there is any amount of THC found in the sample, the charge is made out.

The police do not have to prove that you were impaired to any degree in your ability to drive. And the police do not have to prove that you intentionally drove with THC in your system.

But it is a defence to the charge if you honestly believed that you did not have THC in your saliva at the time you drove and that there was reasonable basis for you to hold that belief. That is, the court must accept that you genuinely believed what you say you believed. The court must also decide that an ordinary person would consider it reasonable to believe that there would be no THC present.

The NSW Government Centre for Road Safety website ( states that the period of time that drugs could be detected “depends on the amount taken, frequency of use of the drug, and other factors that vary between individuals”. The website goes on to say that the saliva test would “typically” detect THC for “up to 12 hours after use”.

A defence of honest and reasonable mistake of fact might succeed where the driver satisfied the court:

  • they believed that the period of detection was up to 12 hours;
  • they obtained that information from the Centre for Road Safety website or a similar official source;
  • they believed that they were a “typical” cannabis user, so that the advice applied to them, and they acted on that advice
  • it was significantly longer than 12 hours (for example 18 hours or more) between the cannabis use and the driving, so that they allowed a margin for error

Once the defence of honest and reasonable mistake of fact is raised, the prosecution must prove beyond reasonable doubt that the driver did not honestly have such a belief or that the belief was not reasonably held.

Obviously, the court would decide on those issues in the particular circumstances of the evidence in each case.