A Wanganui tanning company will appear in court nearly a year after a chemical spill injured 21 of its employees.

The Tasman Tanning Company was charged with failing to ensure employees' safety after a leak of hydrogen sulphide from its Tod St, Wanganui plant on November 2.

Twenty-one staff members were injured, two of them having to be airlifted to Wellington Hospital due to serious injuries, after they were exposed to a cloud of the potentially fatal chemical.

The company is being prosecuted by the Ministry of Business, Innovation and Employment's labour department. It will appear and plead in the Whanganui District Court during a case review hearing on October 30. The case has already been delayed twice due to "administrative adjournments", a court spokesperson said.

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Managing director Hunter Tait told the Wanganui Chronicle soon after the incident the incident would have to be put down to operator error.

"Sulphuric acid was added to the wrong processor at the plant and the combination with the chemicals in that processor caused the hydrogen sulphide cloud.

"Of course, this sort of thing shouldn't happen but unfortunately it can happen," Mr Tait said.