The Defence Department has warned that “highly active” spies threaten Australia's shipbuilding plans.

Foreign agents are plotting to steal Australian military secrets, Defence says.

The threat of international espionage was cited as an official reason by the Department for blocking the public release of briefing notes to the Morrison Government about maritime projects.

Defence made the comments in response to a freedom of information (FOI) application, warning that “Foreign Intelligence Services are currently assessed as posing an extreme threat to Sovereign Capability and Commonwealth Strategic Interests”.

“These adversaries are highly active in pursuing access to information relating to Australia's current and future maritime capabilities in order to advance their own interest and undermine Australian capabilities,” the Department noted in its formal submission.

“Release of this information may be used to directly, or indirectly, damage Australian interests.”

Independent senator Rex Patrick, who lodged the FOI request, says it is almost unprecedented language.

“It's an assessment that goes beyond anything that I've ever heard come from ASIO, Defence or Foreign Affairs,” Senator Patrick said.

“It's clearly a serious national security issue and the Government must do whatever it needs to do to minimise and eliminate this threat.”

Beijing opened a new Consulate-General office in the Adelaide suburb of Findon in 2016, were around 10 staff work on a site alongside the headquarters for the Overseas Chinese Association.

“It hasn't escaped me that the consulate was stood up in the same year that a significant naval shipbuilding program was announced by the Coalition Government,” Senator Patrick said.

Liberal senator Concetta Fierravanti-Wells says similar tactics were used by Soviet spies during the Cold War.

“It comes as no surprise that Beijing has overcompensated the 'diplomatic' requirement to serve in Adelaide,” Senator Fierravanti-Wells told the ABC.

“If ASIO and law enforcement agencies deem laws have been broken, then the 'diplomats' should be expelled.”