Affco workers - including a pregnant woman - have described being evacuated through the path of an ammonia leak on January 11.

The details have emerged in information released by WorkSafe under the Official Information Act.

It includes Affco's duty holder review which it was asked to undertake internally by WorkSafe, which decided not to investigate the incident itself.

The leak - caused when a pressure valve installed just a month before failed to reset - hospitalised three cutting-room workers.

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"I got an instant smell of it and it felt like it was being pushed in my face. I felt a burning in my eyes, face and throat. I went outside hoping to get some fresh air but felt worse outside," one worker said in a written statement.

"The ambulance crew checked me out and gave me oxygen. I felt woozy so they made me lie down."

A sensor detected the ammonia leak at 7.58pm shortly before employees were asked to evacuate to a muster point.

Ammonia was still present there so workers were directed to the old Imlay butcher shop.

It was still too strong but workers has to wait for the plant manager to come and unlock the gate before employees moved to Heads Rd.

"I met a nurse at the hospital and was advised to go for a decontamination shower," the pregnant worker said.

"After the decontamination shower I was put into a room and I was checked over for heart rate, blood pressure, eyes, throat etc.

"The doctor stated that I was okay and all was fine with the baby. They took note that my chest was a bit tight and I had trouble with deep breathing."

Another, who has asthma, described feeling "light-headed".

"I was struggling with my breathing ... I requested an inhaler ... As I was continuing to walk up Heads Rd I stopped to catch my breath. I removed some layers of clothing as I felt hot and was having difficulty breathing."

A shift engineer said it was decided the plant evacuation alarm would not be activated in the boning/chilling room as staff would have "come downstairs and exited straight into ammonia cloud".

Instead, supervisors were contacted via radio and directed away from the ammonia.

The leak was difficult to bring under control because the isolating valve was directly under the leaking valve.

A staff member managed to partially isolate the ammonia liquid while wearing a respiratory mask while someone else damped down the ammonia cloud until Fire and Emergency New Zealand arrived.

Affco has come up with a range of measure following the review including moving the vent pipes to allow for safer access to the isolation valve.

The failing valve has been sent away for a diagnostic report.

WorkSafe has accepted the review and the file is closed.