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Caltex has launched an investigation into the recent fuel mix-up at its service station in Milton.
Motorists filled up with the wrong fuel after the underground 91 octane petrol and diesel tanks at the station were accidentally filled on December 14 with diesel and 91 octane petrol respectively.
The mistake was not noticed until December 17.
Putting petrol in a diesel vehicle can wreak damage.
Jeremy Clarke, spokesman for Z Energy, which markets the Caltex brand in New Zealand, said blame for the incident had not been determined.
"We're undertaking an investigation to understand what has happened, but at the moment our main focus is on looking after motorists who came in during that period and who might be experiencing issues.
"Any motorist who filled up between December 14 and 17, with unleaded 91 or diesel, who has noticed any performance issues with their vehicle, we've recommended they contact the [Milton] site who can provide more details about their next steps."
He said the company was also working with its insurers and created an email address (email@example.com) so any motorist who had not been in touch, could get information.
Mr Clarke said any damaged vehicles would be repaired at the company's expense.