The Cops in Qld and probably throughout Australia take Swabs of saliva from motor vehicle drivers randomly.
I understand those swabs are tested somewhere reasonably in expensively and if positive the driver is charged a month or two later.
It cannot be too difficult a process if the Qld Police can do it
That's not what you're charged and convicted with though. Like failing a breathalyser test, you are taken for further tests.
I believe the roadside swab tests return immediate results in terms of positives but no measure of the amount etc.