Hong Kong authorities on Friday dismissed suggestions that they were too slow to react to a massive rainstorm that put scores of people in hospital and caused havoc across the city, adding that issuing an emergency alert to residents would only have been "stating the obvious".

At a cross-departmental press conference, government officials said the rainstorm that began on Thursday night was a "once in 500 years" event that couldn't have been foreseen, adding that they have been working tirelessly to respond to it.

"[For] typhoons we can make early predictions and preparations. But for this heavy rain... it was so big and so sudden and the predictability was so low. So that's why we couldn't act [as we did with] typhoon Saola... where we could do the announcement very early beforehand," said Chief Secretary Eric Chan.

The observatory's acting director, Lee Lap-shun, said that in the past 24 hours they recorded over 600 millimetres of rainfall – equivalent to a quarter of the usual annual precipitation in the city.

Drainage Services Acting Director Chui Si-kay said Hong Kong's drainage system was simply not designed to withstand such extreme weather conditions and this is why there was such widespread flooding.

Deputy Chief Secretary Warner Cheuk said the authorities did not see any need to warn residents about the situation through a city-wide SMS alert system, used once before during the pandemic.

“In this present case, the onset of the black rainstorm [warning] is very clear. I think any members of the public who are still awake would have noticed this heavy rainstorm situation. So it won’t be necessary to use that system to send an SMS to individual citizens to really state the obvious,” he said.

The Hospital Authority reported receiving 117 injured people while the Black Rainstorm Warning was in effect, including four who were seriously hurt.

A man was found floating in the sea off the Shun Tak Centre in Sheung Wan around noon. He was certified dead after being taken to hospital.

Firefighters received more than 300 reports of floods and dozens of calls about people trapped in lifts. They said they had moved more than a hundred people to safety.

The Highways Department said there had been 70 reports of emergency situations, including subsidence and landslides.

Commissioner for Transport Angela Lee said roads in parts of Hong Kong Island, including Chai Wan and Shau Kei Wan, may be closed for repairs for some time.