The typhoon’s passage prompted the Hong Kong Stock Exchange to resume trading in the afternoon after cancelling the morning session. However, school classes remained suspended for the day.

More than 100mm of rain was reported over most parts of the city since midnight on Sunday, and records exceeded 200mm in some urban areas as well as in Tseung Kwan O and on Lantau Island.

Two flooding cases at the Chai Wan Road Roundabout and Shau Kei Wan Main Street East were resolved in 1½ hours after manpower was deployed, the Drainage Services Department said.

The storm left at least 25 people injured and hundreds stuck overnight at the airport, with long queues at taxi stands stretching back into the arrival hall after the suspension of most public transport.

Connecting bus services were suspended, but MTR trains along open-air tracks gradually began operating.

The stronger-than-expected typhoon left residents frustrated as they struggled to make their way home earlier on Sunday evening, with open-air rail services coming to a halt when the Observatory raised the No 9 warning with only 15 minutes’ advance notice.

The forecaster earlier said the maximum No 10 typhoon signal was a possibility, depending on the strength of winds.

But as the unpredictable Koinu wound south past the city, the Observatory instead downgraded the alert to No 8 at 11.50pm. The government then announced that day classes on Monday would be cancelled.

The string of signals and their timing caught many residents unprepared and unable to get home on Sunday night.

Nicole Lau, a 22-year-old student, was forced to travel to a number of stations in her struggle to return home. She was waiting at Diamond Hill station when the Tuen Ma line train service to Wu Kai Sha was halted.

“I wanted to go to City One Station, but the announcement said all open-air sections had stopped service,” Lau said.

“I then travelled to Kowloon Tong station wanting to interchange at the East Rail line but it was also stopped. So I travelled back to Diamond Hill and there were at least 70 people calling and waiting for taxis.”

She finally arrived home at 9pm after her cousin gave her a ride.

Scientists say strong typhoons arriving earlier, overlapping with extreme rain

Office clerk Karson Cheng, 45, drove his car to and from Nam Cheong station to offer a ride to stranded people.

“Even if you are willing to pay extra on ride-hailing apps, no one takes the order,” he said. “You usually know several hours in advance when the Observatory is to issue a higher signal, but this time it informed us on such short notice.

“Although I understand the MTR was trying its best to protect passengers, it made the announcement too late and the typhoon arrangement is too rigid. It didn’t take into account people who work shifts.”

Desperate Hong Kong parents continue search for missing son as Typhoon Koinu nears

The Observatory warned it might issue the No 9 alert at 6.45pm on Sunday, just 15 minutes before the signal was raised at 7pm.

Garrett Chan, a 46-year-old worker in food manufacturing, said he was shocked by the MTR arrangements as he tried in vain to rush home in Ma On Shan, forcing him to pay HK$1,200 (US$153) for a hotel room in Yau Ma Tei.

“The MTR station was so packed with people, and we supposed it would be more reliable.”

At 6am on Monday, Koinu was centred about 110km (68 miles) southwest of Hong Kong and forecast to move west or west-southwest at a speed of 10km/h across the western coast of Guangdong.

Hong Kong areas hard-hit by last big storm, record rainfall brace for typhoon

At least 25 people have been injured and have sought medical treatment at public hospitals since the typhoon hit the city. They include two cleaners working in Happy Valley who were struck by falling tree branches at around 9.30am on Sunday when the No 3 signal was in force, leaving them with head injuries. The pair were taken to Ruttonjee Hospital in Wan Chai for treatment.

Eighteen reports of fallen trees and a report of a landslide had been received as of 6.40am on Monday.

The Home Affairs Department confirmed that 286 people had sought refuge in 29 temporary shelters opened in districts across the city.

The typhoon also disrupted planes coming and going from the city, with the Airport Authority on Sunday saying about 90 flights were cancelled and another 130 delayed.

“The airport had scheduled 860 flights for the entire day. By 3pm, some 520 flights had departed or arrived,” said Herman Chung Ho-man, the authority’s deputy general manager.

“Most delayed and cancelled flights today were bound for mainland China and Japan, and the average delay time was about 30 minutes,” he said.

Representatives for Cathay Pacific Airways, Hong Kong Airlines, HK Express and Greater Bay Airlines all said operations were running “generally normally” so far.

The MTR Corp said all arrangements made by the company prioritised safety and thanked for the understanding of the stranded passengers.

It added that all stations would do their best to help passengers affected by the typhoon. Such travellers could take shelter at the stations until the weather improved, with water and food provided, it said.

Jockey Club cancels Sha Tin races as Typhoon Koinu nears Hong Kong

Koinu, named after the Japanese term for the Canis Minor constellation, is the second storm this year to trigger a No 9 signal.

Leung Wing-mo, a former assistant director of the Observatory, said Koinu regained some of its strength after passing Taiwan, suggesting that an unusually warmer and wetter northeast monsoon could have revitalised the storm.

“It is like a kid playing a merry-go-round. The monsoon is like a parent who gave a push to the merry-go-round,” he explained.

“The relationship between a typhoon and a monsoon is so complex, making the forecast of typhoons in autumn harder than other seasons.”

Additional reporting by Lars Hamer