The Ministry of Business, Innovation and Employment is investigating the chemical spill at Tasman Tanning.
The incident at the Wanganui factory has left two people in a serious but stable condition in Wellington Hospital's ICU.
They were flown to hospital after yesterday's spill, which also left 19 people exposed to hydrogen sulphide during the incident.
The gas affects the lungs, and in extreme cases it can kill.
A spokesman from the Ministry says an investigation is ongoing, and could take anywhere up to six-months to complete.
Tasman Tanning employs about 200 people and supplies leather to maufacturers around the world.
Photo: NZ Herald