Firefighters discovered the spill at Thermakraft Industries on Turin Place in Otara after responding to an alarm about 9.50am.
The boiling bitumen covered an area about 15m by 25m inside the factory and leaked to an area about 10m by 10m outside, fire service communications shift manager Megan Ruru said.
"The fire service has attempted to contain the spill by covering it with dirt," she said.
"We weren't told of any persons inside the building at the time or whether any persons were hurt or injured.
"A check of the premises was done to confirm that the air inside the building was within the safe exposure limits, which it was, and then we've handed the scene over to the Department of Labour, the city council and the building owners."
It is not known what was inside the building or if any damage was caused by the boiling bitumen.
A pollution response officer from the council is at the scene, where temporary containment barriers have been set up to prevent further pollution reaching waterways.
Ms Blatch said the bitumen, which solidifies "reasonably quickly'', was believed to have been contained.
Ms Blatch said the bitumen that had entered a nearby stream had solidified.
"We don't know exactly how much [reached the stream] but the bitumen is contained and is currently being removed from the stream,'' she said.
By Kieran Campbell - APNZ