It is necessary to perfect the city's national security legal system amid the complex geopolitical situation and hostility from some foreign countries, Secretary for Justice Paul Lam said on Monday, as the government moved to introduce new subsidiary legislation to the city's homegrown security law.

The justice minister said after a Legislative Council meeting that there's a "continuing duty" to make improvements to existing laws.

"In light of the very complicated geopolitical situation in the world, and having regards to actual hostile actions taken by some foreign countries against China and also Hong Kong, there's plainly a need to ensure that our national security legal system will be perfected, will be put in place, so that we can be in a position to address all present and potential national security risks in an effective and proper manner," he said.

Lam did not rule out making further subsidiary legislation to the territory's national security law, saying the legal regime must be reviewed "continuously".

Under the plan, the administration seeks to block off certain locations of the Office for Safeguarding National Security to provide "appropriate protection" to the agency against unauthorised entry.

Briefing lawmakers about the plan, security chief Chris Tang said he doesn't believe members of the public would inadvertently enter sites declared as "prohibited places".

"We will clearly specify the address and the coordinates... The areas within a set of coordinates will be the prohibited place," Tang said.

"It's just like other military restricted areas. There will be clear sign posts erected to say so. Members of the public will not inadvertently enter the [prohibited] place."

The secretary for security also told the special Legco panel meeting that there's no need to declare sites where investigations were conducted as prohibited places.

"When an investigation is carried out, law enforcement agencies will have the duty to provide assistance. There is no need to make subsidiary legislation asking the chief executive to give approval to declare a certain location. It's not in line with our objectives," Tang said in response.

"The objective of [declaring] a prohibited place is to bar unauthorised entry, say for example, a spy going [into the premises] to carry out espionage activities."