The Dairy Workers Union says an industrial dispute at a Waikato factory is getting uglier, with complaints of an assault and sabotage.
Thirty-six union members, about half of the permanent workforce at Open Country Dairy's Waharoa cheese and milk-powder factory, went on strike last week, but the action was later cancelled.
Two days into the strike, up to five cubic metres of sludge was spilled into the Waitoa River.
Open Country Dairy chief executive Mark Fankhauser says police and the company are investigating the spill, as well as an assault complaint laid by one of the workers.
"We've got two full time investigators working on those, so until my concerns are allayed on that I'm unable to allow people back on site.
"That said, because they presented themselves ready fit and able for work, we are standing them down on full pay."
Mr Fankhauser says the company has been given notice by the union that workers will strike again on Saturday.
An Employment Court hearing is set for Friday, when the union will argue that the company is acting illegally by using replacement labour during industrial action.