The owner of Tiwai Point smelter has lodged an appeal against an Employment Relations Authority ruling it must pay workers for back-dated lieu days, saying it could cost the company several million dollars.
The authority told New Zealand Aluminium Smelters Ltd it had to pay workers for time lost, going back six years.
The Engineering, Printing and Manufacturing Union says workers at Tiwai were given eight hours' time in lieu for all the public holidays they worked, despite shift lengths changing to 12 hours, 20 years ago.
New Zealand Aluminium Smelters' acting general manager Stewart Hamilton says the authority's decision imposes millions of dollars of extra costs at a time when the Southland smelter near Bluff is already losing millions each month.
The union says it is disappointed the company is appealing, because the authority's ruling is clear and it is time workers are paid what they're owed.
New Zealand Aluminium Smelters says it hopes the appeal process will be completed by the end of this year.
Australian mining giant Rio Tinto is the majority owner of New Zealand Aluminium Smelters.