A Waikato cheese factory is adamant a disgruntled employee leaked sludge into a river, but the employees' union call that claim 'outrageous'.

Workers at the Open Country Dairy plant in Waharoa have been striking for four days over working conditions.

Environment Waikato had to be called in to the factory yesterday after a sludge spill contaminated the Waitoa River.

Open Country Dairy chairman Laurie Margrain is sure it was done deliberately. He says the company has its thoughts on who did it and knows pretty much when it was done. Mr Margrain says it can be done only by people with very specific knowledge. Police are investigating.

The striking workers say their employer has a callous disregard for the environment. New Zealand Dairy Workers Union spokesman James Ritchie says the accident happened because the company was using untrained staff. He says the union had to notify Environment Waikato about the leak. Mr Ritchie says Open Country Cheese is a company that wants to run production at any cost without appropriately trained environmental workers.

As for the company's claim that a worker caused the leak: Mr Ritchie says it is an outrageous allegation given there is 24-hour security on the factory, which is also monitored by security cameras. Mr Ritchie says it is just not possible for that kind of sabotage to happen.