He was slurring his words my lord; positive test for cannabis at work

There is currently no scientific test which can measure whether an employee is impaired in his or her functions in the workplace from the use of cannabis. Saliva testing is more reliable as to the presence of it in the body than urine testing which might or might not render saliva testing a better proxy indicator of impairment but nonetheless it is not accepted by the scientific and legal communities as a reliable indicator of impairment of function in the workplace. In the absence then of evidence of impairment, an employer cannot assume then that the presence of cannabis through saliva testing is in any way a reliable indication of a risk to health and safety.
Perversely, it is this dilemma which has led the Fair Work Commission to sanction, at least in safety-critical work, specifically aviation, employers relying upon zero tolerance policies to justify dismissals where an employee does present to the workplace and records a positive saliva test for cannabis.
That notwithstanding, a dismissal based upon such a policy breach may be held to be unfair.
Sharps v BCS Infrastructure Support Pty Ltd (2015) FWCFB 1033