Mobil Northgate owner Beven O'Callaghan said 12 customers had experienced problems.
Mr Mills said BP had provided five rental cars to Oamaru customers, and four mechanics were brought in to repair affected vehicles.
Mr O'Callaghan said it became apparent the fuel was contaminated when vehicles that had been filled up couldn't leave the forecourt.
''[The vehicles] that we organised to have repaired are all on the road now, with new filters and fresh fuel.''
A couple of customers had taken their vehicles to other workshops, which were closed for the weekend, and so were yet to get their cars back on the road.
It was very rare for petrol be contaminated, he said.
Any other customers who believed they had been affected could contact any Dunedin Mobil outlet, he said.
Mr Mills said the petrol was believed to have been contaminated on Friday afternoon during routine testing of a pipeline at the industry terminal in Dunedin, which is owned by BP and operated by New Zealand Oil Services Ltd.
The terminal services both Mobil and BP stations in Dunedin and the surrounding area.
A couple of trucks loaded 91 octane fuel from the terminal and delivered it to the affected sites.
The terminal tank was closed at 6.30pm on Friday, the problem rectified and it was operating again about noon on Saturday.
How the water entered the pipe was subject to a full investigation, Mr Mills said.
