At least two people are dead and dozens missing in Taiwan after a mountain dam burst its banks and sent a wall of water crashing into a town during Super Typhoon Ragasa.

The strongest storm on Earth this year was bearing down on China on Wednesday as it barrelled through Asia, having previously smashed the Philippines and caused landslides.

Taiwanese media report that the barrier lake broke and sent torrents of water into Guangfu in Hualien county, washing away a bridge.

Footage shows a black, muddy and debris-filled raging river slamming into buildings, and washing away cars as people cling for their lives.

There were reports of at least 30 people missing.

“In some places, water temporarily rose as high as the second floor of a house and was about [as high as] one floor in the town centre, where the water has been receding,” Lee Lung-sheng, deputy chief of Hualien County Fire Department, told AFP news agency.

“About 263 people were trapped and moved up to higher ground when the river suddenly rose. They are not in immediate danger, but they are very worried about the high water.”

The typhoon will make landfall in China’s Guangdong province on Wednesday (local time).

Earlier, Hong Kong shut down as authorities urged people to stay at home and flights were suspended.

Qantas halted all flights in and out of Hong Kong when the city’s airport was shutdown. 

People piled into supermarkets on Tuesday, leaving little on the shelves, as panic buying set in and residents stocked up on necessities for fear that shops could be closed for two days.

Windows in homes and businesses across the city were taped up, with residents hoping it could help reduce the impact of any shattered glass.

Ragasa, packing hurricane-force winds of up to 220km/h, posed a “severe threat to the coast of Guangdong”, the Hong Kong Observatory said, referring to the Chinese province adjacent to the financial hub.

It will maintain super typhoon intensity as it edges closer to the coast of Guangdong, and as it hits Hong Kong, mainland China and Taiwan, after sweeping through the northern Philippines on Monday.

Hong Kong issued the typhoon signal 8, its third highest on Tuesday afternoon, which urges most businesses and transport services to shut down.

More than 700 flights have been disrupted, including in the neighbouring gambling hub of Macau and in Taiwan.

The weather was expected to deteriorate rapidly later on Tuesday and the observatory said it was assessing issuing a higher warning.

Hurricane-force winds offshore and on high ground are likely in Hong Kong on Wednesday, with heavy rain expected to lead to a significant storm and sea surge in the densely packed city.

Hong Kong authorities warned of rising sea levels, which they said would be similar to those during Typhoon Hato in 2017 and Typhoon Mangkhut in 2018. Both caused billions of dollars in damage.

Water levels will rise about two metres along Hong Kong’s coastal areas – and could reach up to five metres in some areas – the observatory said, urging residents to take appropriate precautions.

Local authorities handed out sandbags for residents to bolster their homes in low-lying areas, while many people stockpiled daily necessities.

At Ragasa’s peak intensity on Monday, maximum sustained winds near its eye topped 260km/h, making it the world’s most powerful category five storm of this year.

It has since weakened slightly but is still capable of wreaking havoc on the densely populated Chinese coast as a category four typhoon.

Chinese authorities have activated flood control measures in southern provinces, warning of heavy rain from late on Tuesday.

More than 10 cities in Guangdong, including technology hub Shenzhen and coastal city Zhuhai had suspended work, transport services and schools due to warnings of storm surges and high waves.

China’s Environmental Forecasting centre said that coastal waters off Guangdong would be hit by huge to extremely rough waves with heights of up to seven metres.

Authorities in Shenzhen have prepared more than 800 emergency shelters, while in the city’s Nanshan district, teams were chainsawing tree branches along main roads ahead of the storm.

Residents in the world’s largest gambling hub of Macau also braced for significant impact, with its casinos shutting on Tuesday afternoon as the former Portuguese colony lifted its typhoon signal to 8.

Taiwan’s government has evacuated more than 7600 people from mountainous southern and eastern areas. Transport disruptions continued for a second day on Tuesday with 273 flights cancelled and some rail services suspended.

-with AAP