Moments after a Dunedin mother placed her baby safely into a car seat, an out-of-control police car slammed into her vehicle, trapping her against another vehicle and leaving her fearing for her life.

Speaking from her Dunedin Hospital bed yesterday with seven fractured ribs, a punctured lung and a lacerated liver, Ashley Rolleston said she thought she was going to die during the harrowing moments she spent trapped between the vehicles.

Witnesses said the police officer was slumped at the wheel of his car as he sped up York Pl in Dunedin before colliding with three cars, a lamp-post and a school bus about 8.30am on Wednesday.

Police say he may have suffered a medical event.

Ms Rolleston said she was returning to her vehicle after visiting the York Pl Preschool and Nursery, where she was interested in enrolling her 11-month-old daughter.

She had just placed her baby girl in the car seat when the crash happened, pinning her between her car and another vehicle.

"Next thing I remember is I was on the ground, pinned on to my vehicle ... I was trying to move but I couldn't.

"I could hear my baby screaming.

"It was terrible, there was one point where I thought I wasn't actually going to make it."

Shortly after, medical students Anna Bergin and Sam Peat arrived on the scene and kept Ms Rolleston calm before an ambulance arrived.

She could not thank the pair enough.

"I just want to say thank you so, so much ... Keep doing what you're doing."

Only a day after the crash, Ms Rolleston was in remarkably good spirits as she waited to learn when she would be discharged from hospital.

"We're just seeing if the hole on my lung has expanded. If it has, I'll have to get a drain put in.

"If not, I should be able to go home soon."

Two police officers had visited yesterday giving her their best wishes and to say they were working to determine the circumstances of the crash.

She expressed concern for the wellbeing of the police officer and hoped he was recovering well.

A police spokeswoman said the officer remained in a stable condition.

The serious crash unit was investigating the incident, she said.