Businesses in the financial hub's coastal areas have been devastated by Ragasa, the strongest typhoon of the year, which killed at least 14 people in southern Taiwan.

A viral video showed waves almost three metres (10 feet) high smashing through the windows of Cholewka's French restaurant, its interior furnishings submerged by churning waters within a minute.

"When you really see it live, it's just devastating," he told AFP on Thursday.

"We don't know how much (seawater) penetrated," he said, adding that his wooden floor was "probably ruined".

The damage to the facilities in his restaurant, in which he invested HK$7 million (almost US$900,000), was extensive.

"Now it's garbage," he said.

Several Hong Kong neighbourhoods such as Tseung Kwan O, where Cholewka's business is based, suffered severe damage despite extensive preparations.

Authorities reported more than 1,200 fallen trees and over 20 instances of flooding and landslides.

Ragasa pushed seawater over Hong Kong's seawalls, overcoming the barriers people had set up in advance and making Tseung Kwan O one of the worst-hit areas during the typhoon.

Pavements were covered in mud and debris, and tiles torn up from the floor had been shattered on the ground by the force of the wind and water, AFP saw.

Tseung Kwan O resident Malcolm Thorp said he was "really sad" about the devastation as he passed by his favourite restaurant.

"I certainly wasn't expecting all of the metal railings, the force that's been applied to smack all of those (down)," the 58-year-old said.

The top typhoon warning was downgraded by Hong Kong's weather service on Wednesday afternoon after being in force for nearly 11 hours -- the second-longest on the city's record.

Globally, scientists warn that storms are becoming more powerful as the world warms due to the effects of human-driven climate change.

Remon Harby, another restaurant manager in Tseung Kwan O, said he has never seen such devastation in more than a decade in Hong Kong.

The 45-year-old Egyptian said he expected he would need at least a month to fix everything.

But with extreme weather occurring more frequently, he said, nobody can afford this sort of "total damage" all the time.

"No business can accept this or can last this way."