The Observatory issued the Amber Rainstorm Signal at 9.30pm on Thursday, a little over three hours after it put up the typhoon signal No 8.

It said the Gale or Storm Signal No. 8 will remain in force and stay in effect until at least noon on Friday.

Tong Yu-fai, Senior Scientific Officer at the observatory, said that Super Typhoon Yagi will be closest to the SAR on Friday morning.

"At 10pm, Yagi was centred about 330 kilometres south of Hong Kong and is forecast to move west-northwest at about 15 kilometres per hour across the northern part of the South China Sea, moving in the general direction of the vicinity of Leizhou Peninsula to Hainan Island," he said.

Train operator MTR said its rail services are now operating at limited frequencies with trains running at 10 to 20 minute intervals, while MTR bus services have been suspended.

It added the last Airport Express train from Airport Station to Hong Kong Station will be extended to about 1.15am to pick up arriving passengers.

Meanwhile, most daytime KMB and LWB routes have also been suspended, but KMB Routes B1 and B9, and LWB Routes S1 and S64 will maintain limited service.

The Labour Department also reminded employers to adopt necessary work arrangements and ensure safety measures for its employees when carrying out electrical work.