BP has admitted the 98 grade supply at its Ōtaki Connect petrol station was contaminated following a delivery on Tuesday, affecting drivers who filled up between 7am and 3pm.
Ōtaki used car dealer Thomas Horton said he filled up a Golf-R at about 10am. He realised something was wrong soon after, as he was driving to Paraparaumu.
"On the highway on the way down, it sort of started doing a bit of a hiccup and a little bit low on power," he said, noting the lack of grunt was unusual for the turbo-powered vehicle.
"On the way back, pulled onto the expressway from Waikanae and then the car just did a really big shunt, kind of like a misfire, and just pulled back all the power. Halfway back from Waikanae to Ōtaki started to hear a rattle under light throttle."
BP said the problem was isolated to the 98 grade at the Ōtaki station during the six-hour window on Tuesday.
However, BP did not answer RNZ's questions about what substance had contaminated the 98, or how the contamination had happened. It also refused to comment on whether the problem had been fixed.
Horton took his car to the mechanics as soon as he got back.
"Kris jumped in the car with me and before we even got to the end of the driveway he said, 'What fuel's in the car?'.
"We got back to the workshop, pulled the fuel line off the rail, and yeah, it was diesel."
Other drivers also complained on social media about experiencing problems with their cars after filling up at BP Connect Ōtaki on Tuesday.
One said their car had a "rattle" after filling up, another said theirs had broken down after putting the new petrol in, while another said they had been starting on a road trip but after encountering the problem had to drain the newly filled tank to empty out all of the fuel. Two people reported a number of motorists had stopped at local mechanics workshops with similar problems.
BP did not confirm when it found out about the problem, or exactly when the pump was closed.
"Immediately upon identifying an issue, 98 supply was made unavailable," a spokesperson said.
However, Horton said he went back to the petrol station at 11:30am, with a sample of the fuel.
"They basically said, 'look it's not us, we've had our pumps done this morning'," he said.
"Then I went back over at about 3 o'clock, after we'd got the car sorted - fully drained the fuel, pushed more fuel, petrol, through the system as well to try and clean everything out, replaced the spark plugs in the vehicle because they were black from the diesel, and then went down to Mobil and then filled up with 98 from Mobil, took it for a drive and it started running okay.
"The response I got was 'oh I don't know what's gone wrong but we don't have an answer for you' and I said 'well you're pumping ... you should have turned them off'."
Horton said he was told the pumps were turned off later that afternoon, after another customer complained. He lodged a claim through the company's website and is yet to hear back.
BP said it was sorry for any inconvenience caused to customers.
"The quality of our fuel is extremely important to us and BP will compensate customers in any instances where there are proven fuel quality issues," its spokesperson said.
"98 grade transactions are being investigated and we are proactively working with impacted customers to resolve this issue. Any customers who are concerned and who purchased 98 from BP Connect Ōtaki during this time period can also contact us on 0800 800 027."
'Could completely destroy the engine'
Mechanic Kris Anton, from nearby Anton's Automotives, said the fuel he drained from Horton's car was yellow - like diesel.
"I serviced his car on Monday and one of the things I did was check the spark plugs and they looked quite clean and new.
"When he came back with the wrong fuel, pulled one of the spark plugs out and it was caked with carbon from not burning correctly."
Anyone who had filled up at the 98 pump during that time should get their car checked out as soon as possible, Anton advised.
"If you've filled up and only put a little bit in, you're probably going to be okay."
However, others who had used more might not be able to restart their cars once they had cooled down, he said.
"Diesel, you have to heat it up a lot more than petrol before it will ignite with a spark, and that could be when people first notice it, is when their car was hard to start this morning, or after work.
"Over the long-term, it could completely destroy the engine to the point that it needs to be rebuilt."
It could be that the wrong fuel had been pumped into the 98 and diesel tanks at the petrol station, or into the fuel tankers themselves, Anton said, noting it would be an unusual situation either way.
"It is something I've heard of before, it's not very common - they do have processes in place to prevent it. I think the biggest thing here is their response to it.
"Blocking [the pump] off is as easy as pushing a button on the computer and it just won't allow 98 to be pumped."
Anton said he had provided BP with a sample of the fuel taken from Horton's car.
Horton said it would be an expensive fix.
The damage to his car, including the mechanic's bill and the discarded petrol, was more than $850.
"We fuel probably 30, 40 cars a month there, so this is the first time that something's happened, but for them to brush it off and keep the pumps open it's not very impressive," he said.
"We're pretty lucky because I caught it within an hour of putting diesel in, you know. Luckily I knew what to do and so did Kris, but the people that were none the wiser that were running their cars all day potentially have some pretty major damage."