Secretary for Justice Paul Lam on Thursday assured lawmakers that any public interest defence allowed for leaking state secrets under proposed domestic national security legislation would come with an "extremely high threshold" and apply only in exceptional circumstances.

During a Legislative Council meeting on the coming legislation under Article 23 of the Basic Law, Federation of Trade Unions lawmaker Stanley Ng queried why there would be any conflict between safeguarding state secrets and the public interest.

Lam said while safeguarding national security is of paramount importance, there can be very "extreme" or "exceptional" circumstances where the safety of a large number of people could be endangered unless a state secret is divulged.

"If we are to consider adding a public interest defence, we would set an extremely high threshold for both the actual circumstances and the presentation of evidence,” Lam said.

The justice minister was also asked about concerns among local clergy that priests may have to notify the authorities if a parishioner reveals during confession that he or she is planning to carry out an act of treason.

Lam said while he believes the chances of this is very low, treason is a very serious matter and everyone must fulfil their duty to inform the authorities if national security is at risk.

Speaking at the same meeting, Secretary for Security Chris Tang sought to reassure organisations that they won't inadvertently interfere with national or Hong Kong affairs merely by having normal ties with foreign groups.

"Even if you work with foreign forces, you may not resort to improper means. For example, you may not resort to violence, you may not cause damage, you may not try to smear someone. If you don't meet this condition, you're not breaking the law," he said.

"And it's also about interference, for example, whether you're interfering with the operation of the Legislative Council and the courts, or interfering with the relations between Hong Kong and the central authorities. So I believe people won't break the law just because they aren't aware that an organisation is related to some state or intelligence institution."

Tang also said prisoners jailed for national security offences could be denied parole if the authorities deem them to be a continued threat.

In general, inmates can see their sentences cut by a third for good behaviour.

However, some people jailed under the National Security Law continued to endanger security or absconded while on parole, Tang said.

"We have made reference to the Terrorist [Offenders] Acts of the UK. In order to safeguard national security, they need to make sure that early release of this prisoner will not in any way endanger national security. And only when this is confirmed will they allow the one-third remission of the sentence," he said.

"We find this concept practicable as far as Article 23 is concerned."