On-the-spot penalty fares for commuters caught without a valid myki are being reviewed amid concerns that the contentious revenue protection ploy coerces vulnerable people into waiving their rights to challenge a fine.

The $75 on-the-spot penalty fares have contributed more than $6.3 million in revenue since Public Transport Victoria started trialling them just 12 months ago, in a bid to reduce paperwork and cumbersome court battles.

Taylor Spence paid a $75 on-the-spot fine after his myki failed to touch on at Huntingdale station. Credit:Eddie Jim

The trial policy is now being reviewed, but new figures show it has already become critical to the state's efforts to combat fare evasion, with about one in three people choosing to pay on the spot since authorised officers began issuing the penalty fares in August last year.

Data from the first 12 months of the scheme reveals authorised officers issued more than 84,000 on-the-spot penalty fares and more than 172,000 $232 infringement notices between August 2014 and July this year.