Lawmakers on Monday urged the government to impose stricter conditions for discharging psychiatric patients from hospitals, after a man with a history of mental illness killed two women in a knife attack earlier this month.

Under the current regulations, patients with a medical history of criminal violence may only be discharged from hospital under conditions like a compulsory medication regime and follow-up appointments.

Roundtable lawmaker Michael Tien said stricter conditions for discharge are needed, such as requiring two doctors to approve the discharge, and ensuring discharged patients take their required drugs.

"We don't have a mechanism to handle people who don't take their drugs," DAB lawmaker Kitson Yang echoed Tien's views, saying more could be done to ensure patients take their drugs regularly.

But Hospital Authority chief executive Tony Ko said doctors have to make decisions on a case-by-case basis.

"Unless the patient is admitted on a hospital order, you cannot compel the patient to receive an injection... In some instances, because of the patient's condition, long-term treatment is required. It's not about a one-off injection," he said.

Acting Health Secretary Libby Lee said the conditional discharge mechanism does not cover all patients at the moment.

"They may have mental health needs and violent tendencies, but they may be voluntary patients in the first place. When these patients are discharged, they are not subject to the conditional discharge mechanism," she said.

"If the family members of the patient are not aware of mental health issues, the support needed and what symptoms to look out for, the patient is not suitable to be discharged to the community."

Following the horrific attack at a Diamond Hill shopping mall, the authorities announced that the conditional discharge mechanism would be reviewed ahead of next month's meeting of the Advisory Committee on Mental Health.

It is one of the ten measures the government announced to strengthen its mental health services.