The Australian Security Intelligence Organisation director-general revealed in his annual threat assessment speech on Wednesday that an ex-politician had been caught up in a spy ring and “sold out their country, party and former colleagues”.

He has since come under intense pressure to name the individual.

In a statement on Thursday night, Burgess said the spy agency was sticking to its long-standing practice of not publicly discussing operational details.

“There are multiple reasons for this, including the need to protect our sources and capabilities,” he said.

“In this case, while we want the foreign intelligence service to know its cover is blown, we do not want it to unpick how we discovered its activities.”

Burgess also said it was an “historic matter that was appropriately dealt with at the time” and the person was no longer a security concern.

“Our democracy remains robust, our parliaments remain sovereign, our elections remain free and the overwhelming majority of our politicians remain thoroughly resistant to even the most sophisticated and subtle approaches,” he said.

Foreign interference laws came into effect in late 2018.

International law expert Professor Don Rothwell said it could be devised that the former politician was active before that time or that authorities didn’t have enough evidence to support a prosecution.

Multiple MPs across party lines, speaking to AAP on background on the condition of anonymity, expressed reservations about retroactively applying the 2018 law despite being outraged at the person’s conduct.

Reasons for this included it was legally dubious to retrofit laws and a person shouldn’t be targeted for following the laws of the land at that time.

But another wanted all levers to be used to ensure the person faced justice and didn’t get away with “betraying their country”.

“There’s a line and this crosses party politics,” they said.

Opposition Leader Peter Dutton publicly expressed support for a discussion about retrofitting the laws.

“The most egregious act is from somebody in public office who betrays their country and I wouldn’t have any tolerance for it whatsoever,” he said.

In the same speech that outed the former politician and exposed Australian academics and a political party insider caught up in the foreign spy network’s web, Burgess flagged previous actions would be covered by current laws.

“Several individuals should be grateful the espionage and foreign interference laws are not retrospective,” he said.

It was also difficult to prosecute anyone under foreign interference or espionage laws, opposition defence spokesman Andrew Hastie said.

“It’s very, very difficult to seek a prosecution for foreign interference or espionage because it requires using classified material which they don’t want to get into the public domain,” he said.

The first person found guilty under the 2018 foreign interference laws was former Liberal Party candidate Di Sanh Duong on Thursday.

He will spend 12 months behind bars after he cultivated a relationship with the then-federal multicultural affairs minister, Alan Tudge, on behalf of the Chinese Communist Party.

– with AAP