Multiple roads were closed, 15 businesses were evacuated and an 800-metre exclusion zone was set up with roadblocks after a piece of machinery overheated at MGA Thermal in Tomago about 5.15am on Friday.

More than a dozen crews from Fire and Rescue NSW (FRNSW) remained at the scene into the afternoon, alongside police, traffic authorities, an Ambulance NSW rescue team and engineering specialists.

An FRNSW spokesperson confirmed about 4pm on Friday that the piece of machinery, which had rocketed from a normal 700 degrees to 1200 degrees, had been stabilised.

A specialist HAZMAT crew will keep an eye on the scene on Laverick Avenue for at least another day to monitor for any flare-ups.

The site was deemed safe on Friday afternoon and all roads were reopened, including Tomago Road.

A bulk carbon dioxide tanker was deployed from Sydney to stabilise the piece of machinery, which was about 14 metres by three metres in size.

Dry chemical powder extinguishers were also used to douse electrical cables that had started burning in the heat.

It was impossible to use water to combat the soaring temperatures due to the risk of it causing an explosion.

FRNSW Superintendent Scott Dodson said it had been an unusual job for firefighters to manage.

"We haven't had many incidents like this before so firefighters will be on the scene for a considerable amount of time just monitoring the plant to ensure safety of the community and other people in the area," he said.

FRNSW Acting Commissioner Jeremy Fewtrell visited the Tomago site on Friday.

Firefighters had been monitoring the atmosphere for smoke but all testing had come up clear on Friday.

The three employees that were inside the MGA Thermal facility at the time were able to escape and alert authorities without suffering any injuries.

Emergency services have reduced an exclusion zone at Tomago as they monitor the temperature of an overheated power plant.

Multiple roads 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.

Tomago Road was closed throughout the morning, but has since re-opened to traffic.

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.

More than a dozen firefighters, as well as engineering specialists from MGA Thermal, remained at the scene on Friday afternoon.

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

They have detected smoke emanating from power cables, though atmospheric readings around the area have been clear of smoke.

FRNSW expects the incident to be protracted.

An 800 metre exclusion zone was put in place, which has since been reduced to 400 metres.

"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."

IN THE NEWS