The Observatory on Saturday issued a flood warning for Sai Kung district as more than 100 millimetres of rain fell in Tseung Kwan O in an hour.

The warning was issued at about 5.25am. The weather forecaster said that with more than 70 millimetres of hourly rainfall fallen or expected to fall, there was a risk of serious flooding.

The warning was updated at 5.40am to note the heavy rain in Tseung Kwan O, with the Observatory saying people should be on high alert

"Residents who are likely to be affected are advised to take necessary precautions to avoid possible flood damage," the Observatory said.

"Heavy rain may bring about flash floods. People should stay away from watercourses. Drivers should beware that there are likely to be serious road flooding and traffic congestion."

The thunderstorm warning was also in effect.