Dan Love's Triumph motorcycle was damaged when a Milton petrol station mixed up diesel and petrol.

Holiday motorists filled up with the wrong fuel after a costly mixup at a South Island service station over the weekend.

A Caltex station in Milton, Otago, accidentally filled its diesel tank with 91 octane petrol and vice versa on Friday, but the mistake was not noticed until Monday.

Otago man Dan Love was one of many motorists hit by the pre-Christmas mix up. He filled up his $25,000 Triumph motorbike with what he thought was 91 fuel, but it was diesel.

GOOGLE MAPS The Caltex service station in Milton, Otago, had a pre-Christmas fuel mix up.

"I got about 5 kilometres and realised the bike was losing power and there was lots of white smoke," he said.

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He managed to get the bike to Dunedin, but it would not start in the morning. The back of the bike, which is only seven months old, was covered in soot and grease.

He is taking the bike to his dealership in Invercargill to see if the engine was damaged by the wrong fuel. He went back to the Caltex station this week to find out what happened.

"They apologised and admitted the mistake," he said.

"They said there were lots of other people affected and said I should refer it to my insurer, which I wasn't happy with.

"If the bike is damaged, I will have to go back to Caltex. I don't think it is fair for me to incur the costs of this."

He said he was annoyed Caltex did not announce the mistake on social media so people were made aware of it.

"The main thing I was annoyed about was there was nothing put out about it.

"People were filling up all day on the Saturday and Sunday. There could be hundreds affected."

Caltex spokesman Jeremy Clarke could not provide figures on how many motorists had been hit by the fuel mix up or how many litres of the wrong fuel had been sold. He said affected motorists should contact the Caltex in Milton.

"We are investigating what happened," he said.

"We know a number of motorists have been unfortunately been impacted by the mix up.

"We are working with them to undertake repairs. The work will be undertaken at our expense."

Christchurch mechanic Keith Reid, of Motor Works, said cars filled with the wrong fuel would stop working after about 5 kilometres.

He said filling a diesel vehicle with petrol could cause damage to the exhaust filters, which could cost up to $6000 to replace. A vehicle filled with the wrong fuel would need the tank and fuel lines cleaned out, which could cost up to $2500 in labour.