He said carbide on its own is not dangerous, but it "reacts very violently with water" and becomes acetylene, which is very volatile.
Great South Rd was closed between Church St and Sylvia Park, and a 500m exclusion zone had been set up.
The spokesman said gas detectors were being used to ascertain the gas levels at the scene.
A BOC spokeswoman said all workers onsite have been evacuated, and the company is working with emergency services to "ensure the safety and well-being of everyone around the site".
A St John spokeswoman said they have had no reports of injuries but are on standby.
She said police had set up cordons in the area.
