Emergency services were called to the incident just off Lunns Rd in Middleton, shortly after 12.30pm on Wednesday.
A Fire and Emergency New Zealand spokesperson said four fire trucks and a hazmat unit attended.
On arrival they found 1000 litres of nitric acid had been spilt.
"Working with police, a cordon was put up around the area. The cordon is still currently in place," the spokesperson said.
"The spill is now contained. We currently have 25 personnel in attendance, who are working to clean up the remainder of the spill.
"They are doing this by applying a neutralising agent. We are expecting to be there for the rest of the day."
A St John spokesman said five people suffered moderate injuries, but none required hospital treatment.
Nitric acid is a corrosive acid capable of causing severe chemical burns.
If inhaled, health risks include corrosion of mucous membranes, delayed pulmonary edema, and even death.
The Fenz spokesman said motorists should avoid the area and police are assisting with traffic control.
The premises is believed to be Owens Transport yard but branch manager Nick Max declined to comment when contacted by The New Zealand Herald.
