About 350 workers had to be evacuated and 11 were treated by paramedics after an elevated level of ammonia was detected within the Hawthorne Street factory about 7.30pm, police said.

Nine of those workers had to be taken to hospitals across the Hunter for observation. A Hunter New England Health spokesman said on Sunday that the workers were taken to John Hunter Hospital, The Mater Hospital and Maitland Hospital as a precaution and did not have to be admitted. 

Bartter Enterprises, a subsidiary of Baiada, released a statement, claiming the plant’s ammonia readings did not reach “levels of harm”. 

“The ammonia leak occurred during installation work by a specialist service provider of new refrigeration equipment outside the factory, with some ammonia making its way into the factory,” the statement said.

“Although the plant’s ammonia readings did not approach levels of harm, our first priority is always the health and safety of workers.

“When some employees said they felt ill, ambulances were immediately called and the incident reported.

“We also enacted our controlled evacuation procedures.” Fire and Rescue NSW Hazmat officers secured the scene and, after the company and authorities tested the ammonia levels, the plant resumed operations. 

“We have commenced an internal investigation and we will also work cooperatively with SafeWork NSW and the EPA,” Bartter Enterprises’ statement said.

It’s not the first time that the Beresfield plant has been investigated for a chemical leak. In October, 2016, Baiada was fined $15,000 for a chlorine leak that hospitalised 42 workers in March of that year. At the time, the NSW Environment Protection Authority said it issued the fine after an investigation found the incident “could have been avoided” after a similar incident occurred at the company’s Adelaide plant three years earlier.

In December, 2016, an employee was taken to hospital after a “small-scale” ammonia leak at the Beresfield poultry factory, the EPA said.