The HA said the man, 63, was waiting to be admitted to Kwong Wah Hospital on the recommendation of a doctor in the outpatient clinic on February 26.
After arriving at the Accident and Emergency Department at around 3pm, he was assessed by medical staff, and was found to be stable and conscious.
The HA also said he had been accompanied by a family member as he was arranged to stay in the waiting area.
But when staff were ready to admit him to a medical ward at 10.30pm, there was no answer when they called out his name, made an announcement on the public address system, or called his mobile phone.
Another attempt to contact him through the PA system was made at 2am, but again there was no answer.
A staff member ultimately found him unresponsive and without a pulse on a stretcher in the waiting hall at 5am. They failed to resuscitate him and he was certified dead at 5.17am.
Kwong Wah Hospital said it had apologised to the patient’s family members and added that A&E department staff had been reminded to enhance monitoring of patients awaiting admission.
The HA said an investigation panel will be formed to look into the incident, and the case has also been referred to the coroner.