Three workers were evacuated from Whanganui's Affco Imlay plant Thursday night after being exposed to a "significant ammonia leak".

Whanganui Fire and Emergency assistant area manager Dave Utumatu was called to the scene on Bryce St about 8.20pm Thursday night.

"We had a significant ammonia leak at Affco and we responded to the second alarm with five fire crews on scene."

St John Ambulance spokeswoman Jess Miller said one worker was taken to Whanganui Hospital with moderate injuries and two others were treated for minor injuries at the scene after the evacuation.

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A Whanganui Hospital duty nurse said the injured worker was treated and discharged overnight.

Mr Utumatu said Affco self-evacuated and did all the right things which was crucial because ammonia was a reasonably troublesome substance.

"It gets into your eyes, throat, nose, ears and everywhere that's been damp. Workers were exposed during the evacuation and the control stage."

He said within the 30 minutes the crews, together with the plant engineer, were able to isolate where the leak was coming from.

"We spent the remaining hour or two making sure there was no pockets of ammonia left on the plant as it can settle in low lying areas and it does take some time to dissipate."

Mr Utumatu said he couldn't comment on where the leak was coming from but during operations all heath and safety measures were taken.

The Chronicle left a message with the Affco plant manager but is yet to hear back.