The Greerton fire fighter hazmat unit was called to Kawerau on Monday night to assist in dealing with a hazardous chemical spill at the Norske Skog pulp and paper mill.

Black liquor, a caustic mix of chemicals used in the pulping process, was leaking from a valve.

Shutting the valve took two men wearing fully encapsulated hazard suits about 25 minutes to shut off, says Kawerau station officer Bill Rackham, but shutting off the valve was able to be done until the support crews were in place.

'It's pretty hot work, it was coming out at high pressure about 14.5 bar and at a temperature of 120C,” says Bill.

The leak of perhaps tens of thousands of litres was contained in a bunded area.

The Greerton Hazardous Materials unit provided the wash down showers for the fire fighters exposed to the liquor.

The Greerton crew were called out at about 7pm and departed Kawerau at about 10.30pm.