Tonnes of molten steel was spilled at the Glenbrook steel mill, south of Auckland.
Hardened metal was used to contain the flow, which occurred at about 10.30pm on Wednesday and burned through two floors of a five storey building.
Fire Service shift manager Scott Osmond says it would have looked quite spectacular as it spread around the ground and it was lucky no one was injured.
A mixture of waste steel and soil was used to make a dam around the molten steel and water was used to cool it.
It was earlier reported that 400 tonnes was spilled.
A statement from New Zealand Steel says only 75 tonnes leaked and it is trying to work out what caused the incident.
It says the full extent of equipment damage is yet to be determined, but it is not expecting it to be significant.
The Fire Service says about 40 fire officers attended the incident to ensure other buildings did not catch fire.
