Sixteen people were taken to hospital on Tuesday morning after inhaling pitch fumes while working on Tiwai wharf in Southland.
New Zealand Aluminium Smelters general manager Ryan Cavanagh says metal was being loaded onto a ship when its pitch tank vented fumes, which drifted towards the wharf.
Mr Cavanagh says some workers coughed and vomited after breathing in the fumes.
All were decontaminated before being taken to Southland hospital.
They have now been discharged and cleared to return to work.
Mr Cavanagh says the incident will be investigated to prevent it happening again.