Wanganui's Tasman Tanning was fined $380k in the Whanganui District Court on Friday after a second gas leak incident at its plant.
It has also been ordered to pay $18,000 in reparations to the victim.
The fine relates to incident in which a forklift driver was exposed to hydrogen sulphide gas in April 2016.
WorkSafes found multiple failings by the company, including a lack of training and warnings, and failure to provide personal gas monitors.
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The company was also fined in 2012 after four workers were exposed to the same gas, hydrogen sulphide.