The observatory downgraded its Black Rainstorm Warning on Monday morning to a red signal as heavy rains brought by Severe Tropical Storm Koinu eased somewhat as the storm system continued moving away from Hong Kong.

The highest black alert had been up for six-and-a-half hours when it was replaced by a red warning at 10.30am. The T8 warning will also be downgraded to a T3 at 11.40am, the Observatory said.

Koinu – which has been drifting westward slowly – had dumped around 200 millimetres of rainfall over most parts of Hong Kong since midnight, with rain levels exceeding 300mm recorded in Wong Tai Sin, Kwun Tong and parts of Hong Kong Island and Lantau Island.

However, forecasters warn that the rain won’t ease completely for some time yet. The Red Rainstorm Warning means more than 50mm of rain could fall over an hour, and could cause serious road flooding and traffic congestion.

"In the past few hours, Koinu tracked westwards steadily. It continued to depart from the Pearl River estuary and weakened. However, intense rain bands associated with Koinu continued to affect the region. Occasional gale winds are still affecting some parts of the territory," said acting senior scientific officer Kong Yu-chau.