Thousands of patients could face delays in getting diagnostic scans after a breakdown at the Lucas Heights nuclear reactor in Sydney.

A conveyer system inside the unit which produces Australia's only source of nuclear medical supplies broke down on Friday, Australian Nuclear Science and Technology Organisation (ANSTO) confirmed.

The facility's production of 10,000 doses of technetium-99m per week for Australian patients has come to a halt.

The products are used in nuclear imaging and are a vital diagnostic tool used in scans for heart, lung and musculoskeletal conditions, cancer and many other medical conditions.

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Engineers were not yet able to "physically attend to the conveyer" an ANSTO spokesman said, although they had been working on fixing the mechanical fault over the weekend.

ANSTO did not give a time frame on when their medicine production line would be up and running, saying it would be fixed "as soon as possible".

The organisation said there were "no safety risks or issues" for staff and people surrounding the southern Sydney facility and the conveyer was "fully contained by shielding".

Emergency supplies being sent from overseas

Sydney's Lucas Heights nuclear reactor has suffered a breakdown. ( ABC News )

ANSTO would not be drawn on whether this would be lead to hospitals having to ration tests to prioritise more serious patients.

"The impact this mechanical fault will have on the Australian nuclear medicine community will depend on matters including their existing stock levels," the spokesman said.

The Sydney facility is the only nuclear reactor in Australia, and one of only 11 nuclear reactors around the world that produce medical supplies.

To try and curb the impact of their supply shortage to patients and hospitals, ANSTO sent bulk supplies of parent isotopes to a nuclear facility in the United States to complete the final stage of the manufacturing process.

ANSTO was waiting for those supplies to be returned so they could be distributed to Australian hospitals.

Dr Ruth Bollard, a council member of the Royal Australasian College of Surgeons, said cancer patients would be prioritised in the event of a shortage.

"I am not aware of any surgical delays from the halt to domestic supply but there could be delays in [diagnostic] assessments," she said.

This is not the first time a fault at the facility has halted supplies.

Not long after the Lucas Heights facilities opened, it was closed again, with loose uranium fuel plates causing technical issues inside the new reactor.

This disrupted manufacturing for months, with Australia reverting to getting their supplies from overseas.

The company expects supplies to be returned from the US and available for patients next week.