An electrical plant emergency in the NSW Hunter town of Tomago has forced evacuations of nearby businesses as firefighters work to defuse the situation.

Fire and Rescue NSW said its hazardous materials units, an aerial platform and a bulk carbon dioxide tanker were at the scene after a “dangerous heat build-up” at the plant just after 5.15am on Friday.

The incident, which occurred in electrical machinery, forced three plant staff and 15 neighbouring businesses to evacuate, FRNSW said in a statement.

Police have closed roads around the facility, which is designed to store heat as a power source, it said.

“Crews in breathing apparatus are carrying out heat readings within the warehouse housing the machinery,” FRNSW said.

“They have detected smoke emanating from power cables.”

FRNSW said it expected the incident to be “protracted”.

– AAP