Fonterra has exported more than 80 tonnes of Gouda cheese contaminated with listeria to the Netherlands, forcing New Zealand trade officials into damage control.

The Ministry of Agriculture, and Fonterra marketing subsidiary NZMP yesterday confirmed that last month Dutch Customs detected the disease-causing bug in a $300,000 cheese shipment from NZMP's Clandeboye factory in South Canterbury.

MAF Food Assurance Authority dairy products director Tim Knox said the serious blunder ended a long run of dairy exports free of the pervasive bacteria.

"It has taken a fair amount of time of the MAF team over the last two weeks dealing with it," he said.

"Our person based in Brussels has probably had daily contact with authorities there to resolve how the product is to be disposed of."

The infected cheese was shipped after a clerical error, a disappointing slip in the trade for the first time "in my six years in the job and some time before that, too", Mr Knox said.

"No one likes errors to occur, particularly one that results in such a significant amount of work and risk occurring," he said. "The risk was that the product got out of control. That didn't happen, which is good, but it's not something that should happen or we would want to happen again."

It was serious for NZMP, he said.

"They've got a reputation that they want to maintain, and they have taken the whole thing very seriously and substantially reviewed their internal systems to ensure the industry rules and our requirements are complied with in all future circumstances like this."

But NZMP compliance manager Mike Willing played down the incident.

The 84 tonnes of Gouda found with listeria was just one day's factory production and was part of an 800-tonne shipment otherwise clear of contamination, he said.

"It is a blip in our performance. Not to my knowledge have we had contaminated product in the market place like this [before]."

"When you are dealing with a volatile product [listeria] is always a major concern and we hope we have robust systems in place but here we had a failure. We've gone back, traced the problem, fixed the problem, increased our testing regime in the plant, looked at other plants that have similar set-ups to make sure the problem can't occur there and are confident we have got it under control," Mr Willing said.

Mr Knox said NZMP alerted MAF to a possible contamination early last month. Further testing by Dutch authorities confirmed the listeria around January 23 when MAF contacted European Commission and Dutch authorities to begin a trace-back and recall process.

Mr Knox said the Gouda was made last February, exported in October or November and arrived in the Netherlands in early December.

More than 90 per cent of the cheese was in a Dutch Customs bond store but a small amount had been processed. It was unclear whether any cheese had got into the market, and Dutch authorities were still investigating. Heat processing of the cheese would have destroyed any listeria, he said.

Mr Knox said inquiries showed that Clandeboye produced a number of listeria-contaminated batches of cheese last February but initial tests indicated the batch eventually exported was bug-free.

The company included it in a list of batches for export approval but MAF regarded it as at risk of contamination and declined to clear it.

"But unfortunately the documentation that was then issued to approve the rest of the material still included this batch.

"Unfortunately that clerical error led to the export being allowed."

Mr Knox said the source of the listeria infection had been traced to a pipe carrying rennet in the cheesemaking process. It was not known whether the bug originated in the rennet, water or another source in the environment.

Listeria bacteria were common in the environment, including in our homes, which made it difficult to combat. The bugs are easily killed by cooking or pasteurisation but the disease, listeriosis, can be serious, sometimes fatal, for pregnant women, babies and older adults.

Mr Knox said most of the cheese had now been destroyed by burning, supervised by Dutch Customs. He described it as "an extensive and expensive exercise for the company".There was no indication European authorities were unhappy with New Zealand's response but "obviously they are unhappy about the event and so would we be if someone was sending it to us."