A fire has been extinguished at an electrical substation at the Lucas Heights nuclear research facility in Sydney's south.

The Australian Nuclear Science and Technology Organisation (ANSTO) said its OPAL reactor was shut down and safe and the fire was isolated to the substation.

It said in a statement that trained ANSTO fire responders put out a fire in the electrical wiring after smoke was reported in the substation at about 10.30am (AEDT) on Tuesday.

Fire & Rescue NSW Superintendent Tom Cooper said three fire crews and a hazardous materials crew were sent to the site where they found the substation was still smoke-logged.

Firefighters and ANSTO crews worked to isolate where the smoke was coming from in a back-up power unit.

"There was a short-circuit in the wiring, they've isolated that now and they're removing it to make sure the whole thing is out," Supt Cooper said.

"It didn't affect anything in the reactor there so that's all good."

Supt Cooper said the fire had nothing to do with the very hot temperatures in Sydney on Tuesday.

ANSTO said it was undertaking an investigation into the cause of the fire and the independent regulator, the Australian Radiation Protection and Nuclear Safety Agency, (ARPANSA) had been notified.