The government is on "high alert" with workers from different units deployed to deal with the effects of torrential rain, Chief Secretary Eric Chan said on Tuesday, as heavy downpours disrupted many public services in Hong Kong.

Chan urged employers to prioritise the safety of their staff and make flexible working arrangements, noting that the conditions in different districts might vary.

The city's weather forecaster issued the fourth Black Rainstorm Warning Signal in eight days at 5.50am. There have never been so many black alerts in one year since the system was introduced.

In a social media post, Chan said he had visited the Emergency Monitoring and Support Centre to find out about the latest weather situation and the various response measures.

"The government is on high alert. The centre has quickly and effectively coordinated with various government departments to mobilise all forces to handle flooding, fallen trees and blocked roads, among other things," he wrote.

Chan said the emergency centre swings into operation whenever a black rainstorm alert is issued, with directorate-level officers monitoring the situation in real time and drawing up responses to minimise the effect on residents.