Western spy agencies are being "overwhelmed" to the point that "bad actors" can "pretty much operate with impunity", according to a former senior CIA operative.

James Olson, who ran the Central Intelligence Agency's counter-intelligence arm, says digital technologies make the traditional "tradecraft" used by agents in the field far less effective.

He says the "Five Eyes" alliance — made up of Australia, the US, Britain, Canada and New Zealand — is being outgunned by countries like Russia, China and Cuba.

"We make some hits here and there and bring some of their operations down, but I think it's just a drop in the bucket," he tells RN's Future Tense.

"I think what they are doing that has not been detected is probably far, far beyond what we even fear."

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The future of espionage looks problematic, with Western spy agencies "overwhelmed" by adversaries and advances in technology.

Professor Olson, who is now at the Bush School of Government and Public Service in Texas, says capacity has become a paramount issue.

"I think the professionals in Western countries are aware of what's going on, but the truth is we are being overwhelmed," he warns.

"None of us have the resources that can match the nature of the threat we are getting from just China alone.

"We're not doing enough, we are not putting enough resources into it, we are not giving enough priority."