The financial hub has suspended school classes and opened temporary shelters, and some hospital services have also been affected.

The city's weather observatory hoisted the "black" rainstorm warning -- meaning downpours exceeding 70 millimetres in an hour -- late on Monday and again in pre-dawn hours.

On Tuesday morning the city's weather observatory warned there could be "severe flooding", adding that hourly rainfall had already exceeded or was expected to exceed 100 millimetres in some regions.

Images circulated on social media showed cars nearly submerged at an outdoor parking lot in the eastern Tseung Kwan O district.

Emergency room service at the Queen Mary Hospital was affected due to "severe flooding" on some roads, health authorities said.

Public transport slowed to a crawl in some districts and a handful of subway station exits have been closed, according to local media.

Eighteen cases of flooding have been identified as of Tuesday morning, according to the Drainage Services Department, adding that 11 had been resolved.

Neighbouring Chinese tech hub Shenzhen issued the "red" rainstorm warning on Tuesday, the first time since 2018, according to media reports.

Tuesday's "black" rainstorm warning in Hong Kong was the fourth in the span of just over a week, beating the previous record of three such warnings in a year.

Scientists warn the intensity and frequency of global extreme weather events will increase as the planet continues to heat up because of fossil fuel emissions.

China is the world's biggest emitter, though it is also a global renewable energy powerhouse aiming to become carbon-neutral by 2060.