"There was a woman standing outside of the car, she looked a bit distressed and concerned so I pulled in front of the car and stopped to see if I could help.
He said the woman was standing, propped up alongside the car, with the father of her baby crouched down beside her, and an older woman.
"I approached them to see what was going on and to see if they needed help, and realised that she was giving birth."
Mr McCollum said he called through to make sure an ambulance was on its way, and went to grab a first aid kit out of the patrol car.
"When I got back she had given birth into the fathers arms. I just asked if the baby was breathing and well and they said that it was. The ambulance arrived shortly afterwards."
He said the woman had been on her way to the hospital, after experiencing labour pains. The family didn't quite make it the whole way.
Mr McCollum has been a police officer for 10 years, having previously spent time as a medic for the army.
Although he had no firsthand experience in delivering a baby, he had undergone delivery training before travelling to East Timor in 2001, he said.
"One of my colleagues he was involved in the delivery of a baby in East Timor."
Mr McCollum said he didn't deserve any recognition, and praised the baby's father for his patience and hard work this morning.
"Having had the training I was prepared to help, but the father did a pretty good job and obviously the baby was healthy and well.
"I think what I've done has been blown out of proportion ... the father, he was right there. If he wasn't there I would have absolutely got in and helped but he's the one that did all the work.
"I just showed up, made sure everyone was okay, and they had the assistance coming in the form ... of the ambulance.
"I would have got involved if the baby had difficulties breathing or was unconscious."
Mr McCollum said it was unfortunate the family didn't quite make it to the hospital in time, and sent them his best wishes.
"It all occurred without too much trouble and no one was too distressed."
A Wellington Hospital spokesman said the mother and her baby were both in stable conditions.
Earlier this month, a woman who gave birth on the side of an Auckland motorway got the chance to meet the cop who played midwife and helped ensure the safe delivery of her first child.
Shanelle Curtis, 19, went into labour five days before her due date last month.
When her contractions became closer together, she left her Otahuhu home with her mother and her sister and headed for Auckland City Hospital, where her midwife was waiting.
However, as they neared the Mt Wellington exit she knew she wasn't going to make the hospital, and her sister spotted a police car and flagged the driver down.
Constable Errol Diprose pulled over and immediately got on his radio to call for an ambulance.
But he knew there was no time and ended up delivering the baby - a healthy boy named Xarien.
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