Another ammonia leak has occurred at chemical giant Orica's Kooragang Island plant in Newcastle.

It's the latest in a string of toxic leaks at the site where ammonium nitrate production was shut down in 2011 because of the problem.

Fire crews were called to the plant at 7.15am (AEST) on Tuesday, about three hours after ammonia vapour escaped from a pipe flange containing 100 kilograms of the liquid chemical.

NSW Fire and Rescue Inspector Greg Symonds told AAP that crews had suppressed the ammonia with a stream of water.

A 50-metre wide safety "hot zone" was established, but air tests had confirmed the danger was well contained, Insp Symonds said.

"We have a zero reading for any ammonia gas from 30 metres away," he said.

"There have been no evacuations and there is no offsite impact."

Fire crews will redirect the liquid ammonia to another pipeline before handing control back to Orica personnel.

WorkCover and the Environment Protection Authority (EPA) have also visited the site, Insp Symonds added.

A NSW Ambulance spokeswoman told AAP that paramedics were alerted to the spill and were on standby, but not required.

A spokesman for Orica confirmed the leak, saying it was below reportable levels but authorities were alerted as a "precautionary measure".

The site remained operational and the cause of the leak would be investigated, he told AAP.

The plant makes ammonium nitrate for the mining industry and ammonia and nitric acid for the industrial and agricultural sectors.