Multiple roads at Tomago were closed and 15 businesses were evacuated after a "dangerous heat build-up" at a power regeneration plant on Laverick Avenue just after 5.15am Friday.

Fire and Rescue NSW said the plant, which stores heat as a power source, normally operates at 700 degrees celsius, but heated to 1200 degrees on Friday morning.

Three staff were evacuated as the incident occurred in electrical machinery, measuring 14 x 3 metres.

A Fire and Rescue NSW spokesperson said a specialist CO2 truck from Sydney was deployed to the scene, as water does not react well to the material. At least five FRNSW appliances, including hazardous materials units and an aerial platform are also the scene.

Crews in breathing apparatus are carrying out heat readings within the warehouse housing the machinery.

They have detected smoke emanating from power cables.

FRNSW expects this to be a protracted incident.

An 800 metre exclusion zone was put in place, which has since been reduced to 400 metres. The Fire and Rescue spokesperson said Tomago Road was re-opening as a result.

"We're working with engineers on site," the spokesperson said. "We're still monitoring air quality in the area to ensure the safety of the community.

"There have been no injuries reported."

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