And they don't know how many people have been affected by another key counterterrorism measure, stripping Australian citizenship from dual national foreign fighters.

Parliament's intelligence committee is reviewing the law that started in 2016 to automatically strip dual nationals of their Australian citizenship if they engage in terrorism-related conduct or are convicted of terror offences.

Government and opposition MPs quizzed Home Affairs officials on Friday as to why the citizenship-stripping power was still needed now the exclusion orders are allowed.

Assistant secretary for counterterrorism policy Derek Bopping said the two powers were used in different circumstances.

"Temporary exclusion orders do not keep people out of Australia," he told the committee in Canberra.

"The temporary exclusion order scheme provides for an individual upon request to be granted a (return) permit by the minister and the minister must grant that permit."

In February, Home Affairs Minister Peter Dutton said 12 people had lost their Australian citizenship under the laws.

The department's head lawyer, Pip de Veau, told the committee there could be more, but the automatic nature meant the government didn't know for sure.

"All we know is the ones that we know about. We don't know the ones that we don't know about," she said.

"There will be circumstances where a person may have lost their citizenship and the government of Australia has no knowledge of that."

Asked whether the department could recommend changes to make it clearer who had lost citizenship, Dr Bopper replied: "At this point, I'd say it's just something we have to accept."

The Law Council of Australia wants the committee to recommend changes to ensure citizenship can only be removed after a person has been convicted of a serious terrorism-related offence, or to have a judge - not the minister - make the order.

National security experts John Coyne and Isaac Kfir, from the Australian Strategic Policy Institute, say there is no evidence the threat of stripping people's citizenship deters them from becoming a terrorist.

In fact, they told the committee in a submission it could be counterproductive because it might stop communities from alerting authorities if they are worried about someone becoming radicalised.

It was far more dangerous to have these people running around the world spreading their views, Dr Kfir told the hearing.

"I would certainly prefer to have them in a secure location where Australian security agents and others are able to work with them," he said.

"We know that some Australians rose very highly within ISIL. There's a senior Australian who now operates as part of their propaganda (arm).

"I would love to get him back because I don't want him over there preaching against us."

Dr Bopper said there was no data on the effectiveness of the threat of losing citizenship because countering terrorism didn't work that way.

Australian Associated Press