A driver's claim that protein powder caused THC to be in his system should not have been accepted by a magistrate without evidence, a court has ruled.
The man was pulled over during random breath testing in August last year.
Police said his speech was slow, and he was seen to have watery, bloodshot eyes and small pupils.
A drug screening test showed there was THC, the chemical responsible for most of marijuana's psychological effects, in his oral fluid.
He was charged with drug driving and brought before the ACT Magistrates Court, where he pleaded guilty in March.
During sentencing, the driver told the magistrate that he never used drugs, and that it must have been his protein powder that caused him to fail the test.
He said the protein powder had "stuff in it" which "obviously tripped over this test", and handed up the packet for the magistrate to inspect.
The defendant told the magistrate he bought it at the Mitchell Produce Markets.
He then asked for a non-conviction order, on the basis that innocent ingestion was an extenuating circumstance.
But the prosecution argued against the non-conviction order, saying the defendant had provided no evidence that the protein powder caused the positive reading.
The driver and the magistrate were then said to have engaged in an exchange about a case reported in The Canberra Times, which involved a muesli product causing THC to be innocently ingested.
The magistrate made a non-conviction order, but prosecutors appealed to the ACT Supreme Court.
The ACT Supreme Court upheld the appeal and delivered its reasons this week, finding the magistrate should not have accepted the man's claims about the protein powder and the innocent ingestion of THC without evidence.
In his judgment, Justice John Burns described the discussion about The Canberra Times article as "unedifying".
"I say that the exchange was unedifying because the scant material put before the Magistrate did not provide any basis for treating the earlier case as factually equivalent to the case before her," he said.
Justice Burns agreed it would be an extenuating circumstance if the man had innocently ingested something that caused THC to be in his system.
But he found the magistrate, after the repeated protests by the prosecution, should have required the driver to produce evidence to back up his claim.
"It follows that the Magistrate was obliged to require the respondent to prove the asserted facts disputed by the prosecution by adducing appropriate evidence," he found.
His decision, handed down on July 17, meant the driver was to be sent back to the magistrate, Dr Bernadette Boss, for sentencing.
The defendant was to be given another chance to prove that the THC may have been the result of innocent ingestion.