In a separate incident, a 17-year-old girl was issued a $75 fine for drinking a can of soft drink while on board a vehicle, before the sanction was downgraded to a formal warning.

But despite the cases of heavy handedness, Canberra Metro has only fined two people for fare evasion since it started charging passengers on May 27.

The fine for not paying the correct fare is $181 for an adult and $75 for a minor, meaning Canberra Metro has raised just $362 in revenue from the offence.

Canberra Metro employs 15 customer service officers, who are trained to check passenger tickets on vehicles and at stops. Inspections, which Transport Canberra refer to as "revenue protection", can also be conducted by police.

A Transport Canberra spokesman has confirmed that around 7.30pm on Saturday, Canberra Metro inspectors conducted a ticket check on a passenger who had a student MyWay card - but no student card or proof of age.

The passenger was told they needed to buy a full fare ticket at the next stop or risk being fined, the spokesman said .

The passenger disembarked at the Dickson stop, before boarding the next northbound vehicle. The spokesman said inspectors rechecked the ticket and handed the teenager an official warning notice.

At that point, the passenger informed the inspector that she was 16 years old, the spokesman said.

"Canberra Metro wishes to apologise to the passenger and family for any distress this incident may have caused," he said.

"Passenger safety and well being is fundamental to our operation and Canberra Metro, following this incident, is reviewing its procedures and response."

On the second incident, the Transport Canberra spokesman said the $75 fine was downgraded after it was discovered that signage advising passengers that eating and drinking was not permitted on the vehicles had yet to be installed.

As of July 1, 415,579 trips had been recorded on the light rail since fare charges were introduced.