The head of the Canadian Security Intelligence Service says he can't guarantee Canada or its allies won't fall victim to espionage again, as it did in the case of convicted spy Sub-Lt. Jeffrey Delisle, the Halifax naval officer who sold secrets to Russia.

Richard Fadden appeared before the Senate Committee on National Security and Defence in Ottawa on Monday and said the Delisle case has led to increased security in many government departments.

"The short answer is, I think our procedures have been tightened. I think they're better," he said.

"Can I guarantee that it will never happen? I surely wish I could, Senator, but I can't."

Delisle was sentenced to 20 years in prison after pleading guilty to one count of breach of trust and two charges of passing information to a foreign entity that could harm Canada's interest.

The 41-year-old man was also fined more than $111,000, equal to what investigators say he received from the Russians in exchange for the secrets. He has 20 years to pay the fine.

"A lot of words have been used to describe the extent of the damage this has done to our own national security and our relations with our close allies. I think what saves us, if that's the right word, in these instances with our allies is that every single one of them has been in the same situation before," Fadden told the committee.

"Having said this, I think there's a consensus amongst ourselves and our close allies that this has been, to some degree, the straw that broke the camel's back. It's caused ourselves and a number of our close allies to review the security arrangements that have been in place within our countries and between our countries."

The story of Canada's most active spy began in July 2007, when Delisle walked into the Russian Embassy in Ottawa wearing a red ball cap and civilian clothes. He flashed his Canadian military identification and asked to meet with someone from GRU, Russia's military intelligence agency.

'The quiet guy who doesn't make a fuss'

Delisle was posted to the security unit HMCS Trinity, an intelligence facility at the naval dockyard in Halifax. It tracks vessels entering and exiting Canadian waters via satellites, drones and underwater devices.

There, he had access to Stone Ghost, an allied computer system. Delisle continued his espionage work until his arrest in Halifax in January 2012.

"Do I think that this is catastrophic? No, it's not. Is it something to sort of say, 'Oh, it happened, it'll go away.' Neither. It's somewhere in the middle. We were hurt in this particular instance as well by not knowing exactly what was passed," said Fadden.

"The technique that Delisle used allowed him to effectively eliminate material transferred after it had been received so we and the RCMP caught a couple of the packages that he was sending and on the basis of that we've extrapolated what he may have sent and it's serious."

Delisle was in debt and his marriage was disintegrating at the time he became a spy. Although such conditions often trigger security reviews for people involved in intelligence, Delisle remained undetected.

Fadden said it's "the quiet guy who doesn't make a fuss" that often succeeds in similar situations.

"If we had had the full series of checks, if we had focused on him, would we have known? He was divorced, he was having financial problems and his family was breaking up. That probably defines a large part of the Canadian population, unfortunately," Fadden said.

"They alone — I don't think — would have been enough for a red flag to have flashed. I guess — in retrospect — I would say maybe an orange flag would have been worthwhile. But he didn't do anything obvious that would lead either ourselves or the Defence Department to believe that he was a traitor. He sort of chugged along, he was a relative quiet guy, didn't make a big fuss."