Hongkong Electric on Thursday said a midnight power outage that hit Hong Kong Island was caused by a short circuit triggered by an accidental connection to a backup cable during maintenance.

The voltage dip on Wednesday plunged several places on the island into darkness soon after midnight for around 45 minutes. Officials received dozens of reports of people being trapped in lifts and fire alarms going off, while some traffic lights were switched off.

Late on Thursday, HK Electric operations director Francis Cheng bowed and apologised to those affected.

He said engineering staff had accidentally connected the electricity supply system to an undocumented backup cable during maintenance. It led to a short circuit that triggered the system's automatic protection mechanism.

"This power outage is deeply regrettable. This is an isolated incident and a very rare one," he said.

"We will handle this in a serious manner as usual, and look in-depth to find out if there is room for improvement in our system."

A general manager with the company, Ip Sung-tai, said the mishap occurred at an electrical station in Cyberport, and noted there was no indication of a backup cable there.

"During the power grid improvement project carried out some years ago, this cable was converted into a backup cable. For more than 10 years, it has been connected to the system, but it has been maintained in a state without electricity," he said.

"Since the data used by the engineers that day did not include this backup cable, the cable handling method was not covered in the on-site procedure.”

Ip said the staff involved had worked at the company for more than a decade.

The firm also said it had taken precautionary measures to avoid similar incidents from happening, and would submit a detailed report in four weeks as requested by the government.