It is understood diesel was found in the tanker that topped up pumps at Bloomfield, Bangor, on Saturday morning.
But the fuel mix-up problem was not discovered until customers left the supermarket.
Stuart McGarry put £50 of what he thought was petrol into his Peugoet 306, less than an hour after the contaminated delivery.
"After I filled up, I drove to Millisle and back, but the car was shuddering