The Black Rainstorm Warning will be downgraded at 5.05pm on Tuesday, according to Hong Kong's weather forecaster, as the city experienced record-breaking rainfall.

"The areas of thundery showers now affecting the territory are departing gradually. The Observatory will issue the Red or Amber Rainstorm Warning signal to replace the Black Rainstorm Warning signal at 5.05pm," it said.

"However, locally there will still be showers, and heavy at times at first. Members of the public are advised to continue to exercise due care. People who have to travel should carefully consider the weather and traffic conditions, and take necessary safety measures."

The black alert was issued at 5.50am – the fourth time the top rain warning was in force this year and the second since midnight – as downpours battered the city and disrupted many public services.

"Until 2pm, 355.7 millimetres of rainfall were recorded at the observatory's headquarters today, breaking the highest daily rainfall record for August since 1884," the forecaster said.

The Geotechnical Engineering Office said it has received 15 reports of landslides as of 2.30pm in areas including Sai Kung, Tai Po, Mid-levels and Lantau.

There were 25 cases of flooding as of 1pm, according to the Drainage Services Department.

Flash floods and landslides had forced a number of roads to close, including parts of To Kwa Wan Road, Bride's Pool Road and Tai Tam Road.

A total of 101 flights have been delayed so far, the Airport Authority said.

Two other flights were cancelled, while another two arrivals had been diverted to other airports.

The Hong Kong Jockey Club said the Mark Six draw originally scheduled for Tuesday evening will be postponed to Thursday "due to adverse weather", with all tickets still valid.