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Fonterra says the potential contamination of 14 milk tankers with mud and gravel is a "very minor incident" and its safety systems worked as intended.
News of the incident, which forced the co-operative to dump 150,000 litre of raw milk, comes as an independent inquiry this week found the dairy giant failed to recognise the huge reputational risk of this year's botulism scare.
Fonterra said tankers at its Hawera milk processing plant in Taranaki were potentially contaminated with mud and gravel on Friday.
About 1-2kg of mud and gravel was believed to have entered the plant's tanker cleaning system from one contractor's truck.
By the time problem was discovered, a total of 14 tankers had potentially been contaminated with water from the cleaning system.
Six of those tankers had picked up 150,000 litre raw milk, which had to be dumped.
Fonterra lower North Island operations manager Scott Walls told Radio New Zealand the problem was found within about 44 minutes of it occurring on Friday evening.
"Those 14 tankers sitting there, all of those were isolated, and six of them had milk in it, which didn't make it into the processing facility."
He said it was a "very minor incident".
"We were able to capture all of it in a very short space of time and the cleaning system is completely separate from our production facilities."
Mr Walls said Fonterra took food safety very seriously.
"That's why we rely on the systems that we have in place down at the Hawera facility, to make sure that these events, if they do happen - they're very rare - but when they do happen, our systems are enacted and we isolate those products very quickly."
Mr Walls said a report the cleaning system had been contaminated with oil and gas drilling waste was part of its ongoing investigation.
It would be another week before all the tests were back.
"What I can tell you is what we saw in the cleaning system on the weekend, which was dirt and gravel. And the investigation team is trying to find the source of how that occurred, and we've isolated it down to one tanker."
Mr Walls said the cleaning system at the Hawera facility was very common in most of New Zealand and the world.
"All of the systems that were in there worked very well and alerted us to the problem, so the automation in the technology, with the follow-up from the staff. So our systems worked really well and captured this problem quickly.
"At the moment, part of the investigation is understanding what went wrong, and that learning will be rolled out to the other sites within Fonterra."