A fire broke out at a nuclear research reactor in Sydney, but authorities said the blaze was brought under control and there appeared to be no damage to the reactors.

The fire broke out as the city sweltered through a heatwave which has triggered dozens of bushfires, but the Australian Nuclear Science and Technology Organisation (ANSTO) said the incident was not linked to the weather conditions.

The state fire service said smoke was detected in an electrical substation at the rear of the Lucas Heights nuclear facility in southern Sydney shortly after 10:30 am (2330 GMT Monday) but the source was yet to be found.

"There was smoke evident in two rooms, they're doing smoke extraction and then they're cooling," a spokesman told AFP.

"Where there's smoke there's fire, obviously, but that's all the information we have at this stage."

Two fire crews and a hazardous materials unit were on the scene but the spokesman said it did not seem to be serious and there was not thought to be any danger to the reactors themselves.

ANSTO said on-site fire wardens "extinguished what was believed to be a fire in electrical wiring", and that it posed no danger to the facility.

Lucas Heights is home to two reactors, the high flux Australian reactor which began operations in 1958 and is in the process of being decommissioned, and its successor the open pool Australian lightwater, or OPAL, which went live in 2006.

The main OPAL reactor was shut down as a precaution and will remain offline until deemed safe to resume operations, an ANSTO spokesman said.

"The OPAL reactor is shut down and is safe. The issue was dealt with within around one hour, no injuries have been reported, and there is no ongoing issue at the site," he said.

The OPAL reactor does not produce power but is used to produce medical and industrial radioisotopes, irradiate silicon used in semi-conductors, conduct mineral analysis and other scientific research.

Temperatures in Sydney are expected to hit 43 degrees Celsius (109 Fahrenheit) Tuesday -- the third-hottest mark in the city since records began.

"ANSTO is currently undertaking an investigation into the cause of the issue although it is not believed that it was linked to current weather conditions," the spokesman said.