Public Transport Victoria has apologised for handing out a fine to a woman with Down Syndrome.

Stephanie, 24, was fined while travelling on a Lilydale bus for not having enough funds on her myki card.

Her mother, Claire Macey, told radio station 3AW the $223 fine arrived in the mail on Tuesday.

"I'm beyond furious. I think it is despicable they picked on Stephanie," she said.

"[Stephanie] has no concept of how to put money on, she just knows to touch on or off her myki."

PTV executive director of customer of customer experience Alan Fedda has since apologised for the fine.

"It sounds like a terrible mistake. We sincerely apologise," he told Neil Mitchell.

He said the incident would be investigated and the fine would be revoked, pending a review. 

Ms Macey, who had been intending to appeal the fine, accepted PTV's apology.

She said the exchange with the ticket inspector had upset Stephanie, as she had been asked to go against her parents' advice and provide details of her home address.

Ms Macey said the myki card Stephanie used was topped up fortnightly and typically carried enough funds.