Taiwan is gradually restoring power supply, the state-run power operator said on Thursday, after much of the island's south and five million households were hit by an outage caused by a malfunction at a major power generation plant.

Hydro and other power plants are being brought on line to provide electricity, said Taipower, which blamed a problem with a transformer at the Hsinta power plant in the southern city of Kaohsiung for a trip at an ultra-high voltage substation.

The major coal-fired station provides about a seventh of Taiwan's power.

"I am very sorry for this major loss of electricity, and am extremely apologetic for the inconvenience caused, especially for the south," Economy Minister Wang Mei-hua told reporters.

Asked whether the plant had been hacked, cabinet spokesperson Lo Ping-cheng said authorities could not rule out any cause.

"It all needs further proof and investigation, which requires evidence."

The cut affected about a third of Taiwan's power supply, hitting about 5 million households, Wang said.

Power should start being restored in southern Taiwan from mid-day, she added, although it was not immediately clear what triggered the problem.

Back-up power supply was at 24% at the time of the incident, Wang said, adding that it was not triggered by insufficient supply, the root cause of major outages in May, when Taiwan was grappling with drought.

The Hsinchu science park, home to many large semiconductor companies such as TSMC 2330.TW, said it did not suffer outages.

The southern Tainan Science Park, where TSMC also has plants, said it experienced a sudden drop of voltage in the morning that did not affect production, however.

TSMC said "power dips" at some of its fabs ran from about 400 milliseconds to more than a second.

"The company is now checking if there is any actual impact," it added.

Some parts of northern Taiwan, including the capital Taipei, also lost power. President Tsai Ing-wen has ordered an investigation and restoration of power as soon as possible, her office said.

A livestream of Tsai's meeting with the visiting former US Secretary of State Mike Pompeo in Taipei has been cancelled, it added.

Normal service has resumed on the high-speed rail line between north and south after three trains were affected, the transport ministry said.

After last year's two major outages brought criticism for the government, Tsai has vowed to scrutinise electricity management.