The new amendments also give customs officials the power to seize items that they deem to “have seditious intention”.
Monday’s amendments ensure that “activities endangering national security can be effectively prevented, suppressed and punished, and at the same time the lawful rights and interests of individuals and organisations are adequately protected”, Hong Kong authorities said on Monday.
Changes to the bylaw was announced by the city’s leader, John Lee, bypassing the city’s legislative council.
While law enforcement officials in many parts of the world have the authority to demand access to electronic devices as part of criminal investigations, the NSL covers a sweeping range of vaguely defined offences from secession, subversion, terrorism and collusion with external forces.
The NSL also allows for some trials to be heard behind closed doors.
The city has seen the arrests of hundreds of protesters, activists and former opposition lawmakers since the introduction of the NSL.
In February, the father of pro-democracy activist in exile was jailed for trying to cash out her insurance policy, under a homegrown law that expands on the NSL.
Also in February, media tycoon Jimmy Lai was sentenced to 20 years in jail after being convicted of foreign collusion and publishing seditious material under the NSL.