Blackened skies unleashed torrential rains on Hong Kong and the high-tech cities surrounding south China's Pearl River Delta on Tuesday, shutting hospitals, schools and law courts, and turning the Asian financial centre's many staircases into waterfalls.

Around 10,000 lightning flashes lit up Hong Kong's skies between 6 a.m. (2200 GMT) and 6:59 a.m., according to the city's weather authorities, as up to 90mm (3.5 inches) of rain per hour continued to drench the city and neighbouring Guangdong province.

Videos showed torrents of water cascading down steep hillsides in the former British colony, breaking into white-water streams as they rushed down staircases linking Hong Kong's multi-tiered cityscape.

The deluge prompted the weather bureau to extend its highest "black" rainstorm warning until 3 p.m.

The water had risen to ankle-height outside Hong Kong's largest hospital, as medical authorities announced they would be closing clinics across the city due to the downpour.

Extreme rainfall and catastrophic flooding, which meteorologists link to climate change, pose mounting challenges for officials in mainland China, sweeping people to their deaths, displacing thousands, and threatening billions of dollars in economic losses.

The storms follow deadly flash floods in Southern China over the weekend, which left five dead in Guangdong province and prompted a large-scale search operation involving over 1,300 rescuers.

As of Tuesday morning, four rivers in the province had risen to such a height that were at risk of bursting their banks, China's state broadcaster CCTV reported.

Hong Kong, Guangdong Province, and Macau - which issued a Red rain warning - form the backbone of Chinese President Xi Jinping's flagship Greater Bay Area initiative, aimed at fusing Hong Kong's financial firepower with Guangdong's manufacturing and technological strength.

Airports across the region reported cancellation rates of around 20% on Tuesday, according to data from Flightmaster, while speed limits on the Hong Kong-Zhuhai Macau Bridge - a flagship GBA infrastructure project - were lowered due to poor visibility.

Hong Kong's airport has maintained normal operations throughout but said some flights had been delayed and passengers were advised to head to the airport only after their flight time was confirmed.

The Hong Kong Stock Exchange remains open, having changed its policy to continue trading whatever the weather late last year, but the judiciary announced that courts, tribunals and register offices would remain shut until at least two hours after the 'black' rainstorm alert was cancelled.

"Persistent rainstorm will cause serious road flooding and traffic congestion. Members of the public are advised to take shelter in a safe place," the Hong Kong Observatory said in a bulletin on its website.

Hong Kong typically receives an annual average of 2,200mm of rainfall, more than half of which usually falls from June through August.

But it is not all doom and gloom.

Hong Kong Disneyland announced it remains open, and its daily "Friendtastic! Parade" is still scheduled to take place later today, according to the park's website.