Detailing the process for the first time, Immigration Department head Mike Pezzullo​ said public servants and officials would advise the Immigration Minister if they thought someone had carried out terrorist activities justifying the loss of citizenship.

The issue goes to the vexed question of who decides that a person is a terrorist deserving of having their citizenship stripped, given no court conviction is needed under some parts of the proposed new laws.

Under extensive questioning by Labor MPs in the parliamentary joint committee on intelligence and security, Mr Pezzullo said public servants and officials – probably intelligence officers, police and bureaucrats – would prepare a brief of advice for the minister if they believed a person's terrorism activities cross the threshold for loss of citizenship.

The minister would then issue a notification of the citizenship loss, at which point it would take practical effect, such as having the person's passport revoked, being taken off the electoral roll and losing welfare rights.

Asked by shadow attorney-general Mark Dreyfus whether the entire process would be "in secret", Mr Pezzullo said: "I think it would all be done in secret, Mr Dreyfus ... We would do it confidentially, yes."

Technically under the legislation, the person "renounces" their own citizenship as soon as they carry out certain terrorist activities. But this takes practical effect only when the minister issues the notice of the renunciation.

The Commonwealth Ombudsman has described this process as "a legal fiction".

The legislation was structured this way to avoid giving the discretionary power to the minister to strip people's citizenship, which experts widely agreed would be unconstitutional.

But Labor MPs – whose support will ultimately be needed to pass the new legislation – expressed concern in the inquiry that this meant the decision would instead be taken by officials.

Senior Labor frontbencher Penny Wong said: "You have to decide that the facts have been established … it's not like in the ether is it?"

Mr Pezzullo answered: "If it assists the committee, I think Senator Wong's characterisation in that 'small-d' sense of 'decision', yes, a group of officials have to decide to do their job diligently."

Department officials also confirmed that a person could lose their citizenship based on intelligence advice even if they had been acquitted by a court of criminal conduct for the same activity.

Mr Pezzullo also confirmed that the minister would not actually be obliged to tell a person they had lost their citizenship, nor give reasons for the loss.

Mr Dreyfus said the committee had been told by multiple legal experts that the legislation in its current form would be unconstitutional.

The government has refused to release the Solicitor-General's advice on the legislation. But Mr Pezzullo said it was "on legally sound ground".

With Jane Lee