The Hong Kong Observatory issued the Black Rainstorm Warning Signal at 4pm on Wednesday, before downgrading it to a red warning.
The red signal indicates that heavy rain has fallen or is expected to fall generally over Hong Kong, exceeding 50 millimetres in an hour.
Earlier, the Observatory advised the public to take shelter and remain indoors.
The landslip warning and thunderstorm warning are also in force, and there is also a risk of flash flooding in the northern New Territories.
Some readers have reported flooding around Tai Tam Road on Hong Kong Island.
Other have reported that cab hailing app Uber is charging up to five times the regular amount for a taxi.
s black rainstorms pic.twitter.com/OZYiREQn3B
— David Watkins (@DavidJWat) October 19, 2016
Super Typhoon Haima is moving towards the northern part of Luzon in the Philippines.
It is expected to enter the northern part of South China Sea on Thursday.
Haima will edge close to the coast of Guangdong on Friday, the Observatory predicts.
Additional reporting: Tom Grundy.