An urgent health warning has been issued after a burst pipe at the Westland Milk plant in Hokitika released a potentially harmful liquid onto a nearby public walkway.

A chemical cleaning hose at the factory's rail siding burst on Sunday afternoon, resulting in an uncontrolled chemical release of hot water vapour, which spread across the nearby walk and cycleway. 

Westland Milk communications manager Steve Attwood said for a period of the time the vapour included dilute nitric acid.

The company has issued a notice that anyone who was on the Hokitika river bank cycleway behind the company's Hokitika Factory between 1pm and 4:30pm on Sunday is advised to seek medical attention in case they have come into contact with a potentially harmful liquid.

"Anyone who came into contact with the liquid vapour while acid was present might experience skin rashes, sore or damaged eyes or have respiratory irritation if the vapour was inhaled," he said.

People who believe they have been in contact with the vapour are advised to see their own doctor, local A&E or call the chemical hotline on 0800 243 622 for advice. 

They should explain they might have been in contact with "ACIDBLEND N600" in dilute form.

Attwood said the accidental release of the liquid occurred between 1pm and 3pm approximately but, as a precaution, anyone in the area up to 4:30pm should get medical attention.

"Even if someone who came in contact with the spray is not showing symptoms, they are best advised to seek medical advice," he said. 

He said Westland Milk had notified the Grey Base Hospital accident and emergency department and Hokitika medical centres.

It had also notified WorkSafe, local authorities and launched an internal investigation into what happened and how to ensure there was no repeat.