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Westland Milk Products say a chemical cleaning hose at its factory next to the Hokitika riverbank cycleway burst at about 1pm this afternoon.
The burst pipe released steam, which for a time included dilute nitric acid, the company's communications manager Steve Attwood says.
He warns anyone who came into contact with the substance could develop skin rashes, sore or damaged eyes, or have respiratory irritation if it was breathed in. And anyone who was on the Hokitika riverbank cycleway behind the Westland factory between 1pm and 4.30pm this afternoon, should seek medical help.
Westland has also notified Worksafe and local authorities about the incident, Mr Attwood says.
"[It's] too early to speculate on cause. What happens with these situations is that the company itself launches it's own investigation, but we also report it through to Worksafe, and they've already informed us that they intend to have people on site as soon as possible.
"So essentially there'll be two investigations running, our own and Worksafe's and we'll be cooperating with Worksafe of course."