The Public Transport Authority says the professionalism of its transit officers has been unfairly besmirched after a passenger who complained of witnessing oral sex on a peak hour train admitted her story wasn't true.

The incident rocketed to the top of Perth's news agenda last week after a passenger claimed to have witnessed fellow passengers indulging in the lewd act on the Armadale line.

Kerry told 6PR she noticed the lewd act on the Armadale line during peak hour.

Last week, she told Radio 6PR that transit officers on the train had failed to put a stop to it or investigate her complaints.

But the PTA says that after reviewing more than 100 hours of security vision footage it failed to find any evidence that it happened at all.