Two off-duty ambulances were heavily damaged in a fire at 4.30pm on December 30.
St John Central Otago operations manager David Baillie said, following investigations by Fire and Emergency New Zealand, he could confirm the fire at the Cromwell station was caused by an uncommon electrical fault.
The initial police report had said the fire was suspicious, which raised concerns on social media that an arsonist could be targeting emergency services in the area.
Mr Baillie said St John took the health and safety of patients and ambulance staff very seriously, and regularly conducted maintenance and service checks on all of its vehicles.
"We would like to reassure the Cromwell community that the main Cromwell ambulance is still fully operational and available to serve the region," he said.
"Other resources and vehicles are also already available and fully operational in the short term.
"There is no impact on our service in the Cromwell community and surrounding areas."
