Four Pan Pac Forest Products workers required medical treatment yesterday following a gas emission from a kiln at the Whirinaki site.
The kiln was the same one that had been shut down in April 2015 because of issues with emissions safety and health concerns.
Pan Pac general manager Doug Ducker said there had been an operational failure on the kiln, and the staff members had been subjected to gases emitted from the kiln which were normally burned. He said there had been a limited period of exposure and staff "registered with effects" were treated.
Three people were taken to the Napier Medical Centre and one to Hawke's Bay Hospital.
A hospital spokesperson said the person had since been discharged.
Mr Ducker said the company would re-start operations once it had full clearance. Yesterday's incident was the fault of a particular valve.
In February 2015 the thermally modified timber plant was shut down because of rashes and other ailments which staff and neighbours feared were caused by emissions from the new timber heating system.
The discovery of engineering issues meant the kiln was shut down in April 2015 for all but testing purposes.
In August, authorities said tests showed levels of air pollutants were well below recommended guidelines when the kiln was operating normally.
But they suggested acrolein and acetic acid levels could exceed standards if a burner used to eliminate noxious gases stopped working during the kiln operation.
The kiln had been operating for the past few months with no issue, Mr Ducker said.
The $2.5 million kiln was introduced in April 2014.