Defence officials told a parliamentary inquiry hearing on Friday the Australian Signals Directorate lost electricity from ActewAGL, and back-up generators failed to power one of its two buildings for two hours during the outage.

The ASD is relying on diesel generators needing parts no longer manufactured to prevent a power shut-down, Defence warned.

It said the agency, which counters cyber espionage, risks power failures that could jeopardise government security unless three 50-year-old generators are replaced in a $75m overhaul of its back-up energy and cooling systems.

Defence's director-general of capital facilities and infrastructure Brigadier Noel Beutel told the parliamentary committee inquiry into the proposed works the generators were past their design and operational life, had asbestos and contained discontinued parts.

The spy agency had to source specially manufactured parts to replace speed controllers for its generator system.

A power shut-down would stop its 1900-strong workforce from intercepting foreign communications and providing intelligence to the Australian government and military, and would cut off the nation's secure communication links, the committee heard.

Defence also warned in March the nation's cyber security was put at risk when it was asked to help with load shedding during soaring temperatures on February 10.

The ASD shifted to generators as a precautionary measure because it was concerned about the reliability of the grid.

"The biggest issue that we have with the existing diesel generators is in relation to reliability," Brigadier Beutel said.

"Because of the nature of the operations at the ASD, it is classified as a building that has to have 100 per cent assurance of redundancy because of the critical nature of the work that's performed in that building."

Its Russell buildings would be fit to house the agency for another 20-30 years if the project was approved, officials said.

The upgrade would give the spy agency a new diesel generator house, reconfigured high voltage electrical equipment, new electrical infrastructure, and an upgraded cooling, heating and air distribution system for communications and server rooms.

The works, if approved, would begin in mid 2017 and finish three years later.

Another option, constructing a new building, would cost $800m.

The agency has approval to grow by up to 460 personnel over the next 6-8 years.

While a new building was not an option in the near future, the ASD preferred this in the longer term, Defence director of facilities and data centre services Jim Mapletoft said.

The ASD, whose mission statement is "Reveal their secrets - protect our own", relies on high-tech computing, and is thought to be extremely energy intensive.