"I was yelling out as I went 'stop, stop, stop that man, he's a thief', and I was really, really pleased with the public response actually; they were great."
Ms Manderson said when she reached the offender's vehicle she saw he had a handgun and she backed off.
"I was with another gentleman, he opened the car door ... the offender had a firearm so that sort of put me off pretty quickly," she said.
"The gun was on the seat of the car and he had his hand on it and indicated to it."
Ms Manderson rang 111, giving details of the vehicle and offender.
Police said this morning a man was arrested in Porirua overnight in relation to the incident.