An ammonia leak brought major disruption for motorists near a popular South Island holiday town today.
The gas alert at the Alliance lamb and sheep processing plant at Stoke cut road access on a key route for a couple of hours.
Just before 10am a safety valve at the plant near Nelson failed, releasing the hazardous gas into the air.
Fire Service incident controller Tane Simpson said it was a large leak given the size of the plant and they lost about 100 kilograms of liquid.
The incident sparked a sizeable emergency response, drawing in 30 firefighters from surrounding areas.
A large cordon was put up and all access routes between Nelson and Richmond, including the motorway, were shut for about an hour .Although a detour was put in place, it took a long time for the backlog to clear.
"Sometimes we just have to quickly close it down so that we can ensure safety of everybody around," Sergeant Mike Wardle said.
"Everybody was slightly annoyed but co-operative."
Twelve maintenance staff working at the plant were evacuated but the company is downplaying the seriousness of the incident.
"Everyone was checked by ambulance staff and all were fine," Alliance plant manager Terry Kreft said, adding he believed only a small amount of gas escaped into the atmosphere.
The leak was quickly isolated but a northeast wind made the situation difficult for fire crews.
"The ammonia was quite strong in the area we were so we repositioned ourselves upwind so we were out of the path of the ammonia," said Simpson.
Alliance is investigating why the valve failed and the plant is set to reopen for business next week.