Democratic Party lawmaker Lam Cheuk-ting says giving mainland authorities up to 30 days to detain Hongkongers without anyone knowing in Hong Kong is far too long.
He was speaking after the government announced details of a revised notification system for when residents are detained on the other side of the border for committing a crime.
"I also worry that whether the definition of national security is clear enough for Hong Kong people," Lam said.
"We all know that in mainland China they can redefine the meaning of national security for any political reason."
In Beijing, Security Secretary John Lee was asked what constitutes a complex case or an offence that endangers national security. He would only say that every case is different.
"Obviously each case has its own circumstances and its scope of an investigation," he said. "As regard what will be contravening national security, this is in accordance with the laws on the mainland."
Legco’s security panel chair. the DAB's Gary Chan, said the new arrangement is more transparent and he believes it can allay the concerns of Hong Kong people.