Whanganui's Affco Imlay plant has come forward with more details about last week's significant ammonia leak after an outpouring of fury hit social media.

Last week the Chronicle reported that three workers had been hospitalised with moderate injuries after being exposed to the toxic gas at the Whanganui plant.

Concern rose when some of the workers began commenting on the news story on Facebook, suggesting the incident was much worse than reported.

Read more: Three Affco Imlay workers exposed to toxic gas

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Renz Thomas said: "We were all exposed. It was a major leak we could barely breathe and see....we were trapped."

Paris Schlegel said: "I feel sorry for the pregnant lady, what if this affected her baby! Or even killed all the others, they should do more to look after there workers."

Jordy Kahl said: "What sucked the most is they didn't send us home they just made us carry on working."

Affco general manager Nigel Stevens confirmed that over 200 people were evacuated, three workers received medical treatment and one of those workers was pregnant.

"The worker [who was pregnant] expressed concern over any effects of potential exposure to her supervisor, who as a precaution directed her to the ambulance for assessment.

"She was taken to hospital to be assessed and released that evening with no treatment required. She returned to work the following day and there were no ongoing effects," Mr Stevens said.

Two woman, 20 and 25, and a 23-year-old man received medical treatment but all were discharged from hospital that night.

"The three people that were assessed did not return to work that night. The plant restarted following an all clear from the shift engineers and fire service in attendance."

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Mr Kahl said that leak came from an engine room area following a release of ammonia when a safety valve failed.

"An o-ring within the safety valve became unseated and did not seal. The safety valve was certified and was put into service in December 2017."

He said all health and safety measures were taken.

"Plant evacuation was completed with all workers accounted for following evacuation. Emergency services were available and active in dealing with the call out and treatment required."

A Work Safe spokeswoman said inquiries were being made into the ammonia leak but it has not been progressed to an investigation yet.