Workers involved in an industrial dispute at a Waikato dairy factory have been told by their employer not to return to work on Monday evening.
Thirty-six union members, about half of the permanent workforce at the company's Waharoa cheese and milk-powder factory, began strike action last week over negotiating a collective agreement.
They voted to go back to work on Monday night after the Employment Court granted them an urgent hearing on Friday to determine whether the company is breaking the law in their use of replacement labour during industrial action.
The dispute has turned increasingly bitter in recent days following a sludge spill at the Waharoa plant on Friday night, which Open Country Dairy believes was sabotage committed by unionised workers.
In a letter sent to the Dairy Workers Union, the company says the 36 workers can't return to work as other arrangements are in place.
The letter also says other serious incidents of sabotage have also taken place both before and during the strike.
Unionised workers, who are now back on full pay, are being told they might be able to return to work on Tuesday at 6pm.
But this will be dependent on ongoing investigations by Open Country Dairy and the police.
Police and Waikato Regional Council are investigating a sludge spill into the nearby Waitoa River.
Open Country chief executive Mark Fankhauser says the spill was caused by someone deliberately opening a valve.
Between Friday night and Saturday morning an effluent treatment pond overflowed, sending sludge into the Waitoa River.
The company has not directly accused the Dairy Workers Union of sabotage, but does not believe the spill was caused by anyone who was legitimately on the site.
The union says 24-hour surveillance means there's no way a member on strike could have sabotaged the operation.
It blames untrained personnel for not managing effluent ponds properly. The company disputes that.