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One car was found alight in the secure police station car park - another one parked next to it was also damaged.
Both vehicles were damaged beyond repair, with police earlier labelling the fire as suspicious.
Canterbury Metro Commander Superintendent Lane Todd said further enquiries had established the fire was caused by an electrical fault.
A fire investigator worked alongside detectives since first light to conduct the scene examination.
"One of the vehicles was well alight, while a second vehicle parked next to it was also on fire.
"It was quite an extensive fire."
Supt Lane Todd earlier said police were "very concerned about the incident. The vehicles were in a secure yard. It's one of our 24-hour stations."
Police have thanked the community for their support during the investigation.
- Additional reporting NZ Herald