As he drove down Heretaunga St, he saw what seemed to be two men leading a woman towards an alley and U-turned to investigate.
"You never know what you're going to find," he said. "Usually it will turn out to be someone urinating against a wall or a couple of people fighting. This time I found this offender ... it was obvious what was going to happen."
He called for back-up and the woman got to her feet and stumbled out of the alley to be met by another officer arriving at the scene, as Mr Stewart "rounded up" the man and an associate who was in the area.
"The first thing was to make sure she was safe, and I'm really happy about being there at the right time," Mr Stewart said.
"This is the result, but it could have been worse."
This week 25-year-old Mathieu Batick, from Vanuatu, was sentenced to two years and seven months' jail. Without any previous convictions, he had denied a charge of indecent assault and one of assault with intent to commit sexual violation, but a jury found him guilty after a Napier District Court trial in August.