The Tahmoor dispute follows industrial action and a lockout at the Bulga mine near Singleton, and the mineworkers' union said further industrial action at other Hunter mines could not be ruled out.

United Mine Workers union district vice-president Greg Sharp said yesterday that negotiations were soon to start on new enterprise agreements at two more Xstrata operations, the Glendell mine north-west of Singleton and the nearby Liddell coal preparation plant.

Mr Sharp said the coal industry had made booming profits in the past few years and wages had nowhere near kept pace with earnings.

Even more importantly, the union wanted to wind back some of the industrial changes made under John Howard's WorkChoices laws.

One example of this was at Glendell, where the workers were employed on common-law contracts, which the union wanted replaced with a collective agreement.

Mr Sharp said the Bulga dispute was as much about conditions of employment as it was about money.

He said Xstrata was pushing for a clause in the Bulga agreement that meant arbitration would only take place if both parties agreed.

"That means that if the company didn't want something arbitrated, it would never be settled," Mr Sharp said.

"Arbitration is not about dragging the company off to court for any little thing, as Xstrata has accused us of doing.

"Rather, it's about having an enforceable agreement. The only way an enterprise agreement can be properly enforced is to have access to an independent umpire to make binding decisions when disputes arise."

Xstrata spokesman James Rickards said a number of issues remained to be settled at Bulga and the company could not rule out further action.

Xstrata's lockouts of workers are known as "response actions" under Labor's industrial relations regime and Mr Rickards said locking out employees was "a fair and measured response" to continued strikes and stoppages.