"One of the vehicles was well alight, while a second vehicle parked next to it was also on fire," he said.
The fire was initially treated as suspicious but inquiries today by Fire and Emergency NZ investigators and police determined the blaze was caused by an electrical fault in one of the cars.
"It is believed this was an isolated incident with the vehicle involved being an older vehicle in the fleet.
"Police would like to thank the community for their support during the investigation today."
A fire investigator specialist had worked alongside detectives to conduct a scene examination since first light, Todd said.
"[Firefighters] attended and put the fire out. This morning [at] first light we have had a fire investigator specialist with detectives doing a scene examination.
Earlier on Sunday Todd said police were very concerned about the incident.
"The vehicles were in a secure yard. It's one of our 24-hour stations.
"It's a concern for us that something like this could occur. However, it is early days in the investigation and we have to keep an open mind."
He said police would pull CCTV footage and speak to potential witnesses as part of the investigation.
