Molten glass is leaking at a factory in the Auckland suburb of Penrose.
Nearly 30 firefighters and five fire engines are on the corner of McNab St and Great South Rd after the leak was reported at 5.40am.
Further details are not yet available but a number of factories are in the surrounding industrial area.
A Northern Fire Service spokesman said firefighters using 10 hose reels were helping site management cool down the liquid glass.
"We'll be here for some time," he said.