The Australian Services Union says strike action by Hunter Water employees at Newcastle's sewerage treatment plants could see increased levels of coastal pollution.
Twenty eight employees have walked off the job over plans to cut staffing at the plants over Christmas from two to one.
ASU Secretary Sally McManus says a solo worker at a Melbourne treatment plant died recently and was not found until a week later.
She says a refusal by members to work could have dire environmental consequences.
"Sewerage treatment plants obviously deal with outflows, they deal with environmental issues," she said.
"They're essential to the operation of everything, so this is a really bad situation that management's put everyone on in.
"We're just asking for them to keep the status quo and have two people working until we're in a position to say it's safe not to do that."
Hunter Water says it is in ongoing discussions with the union and the safety of employees and the public is their number one priority.
It has assured the public the treatment works will continue to operate effectively and safely.
Australian Broadcasting Corporation