The Standby Signal No 1 took effect on Wednesday evening as Super Typhoon Doksuri moves closer to Hong Kong.

The No 1 signal was issued at 8.40pm, indicating that a tropical cyclone is within 800km of Hong Kong.

The Observatory said as of 9pm, Doksuri was about 720km east-southeast of Hong Kong, and is forecast to move northwest or north-northwest at about 12 kilometres per hour across the Luzon Strait, entering the northeastern part of the South China Sea.

It's heading towards the coastal areas of eastern Guangdong and southern Fujian.

The Observatory said the super typhoon will be more than 500 kilometres from Hong Kong Wednesday evening to Thursday morning, and the No 1 signal will remain in effect during that period of time.

Forecasters said chances of strong winds over the SAR are relatively low, but there will be swells and members of the public should stay away from the shoreline.

They also said very hot weather will persist over southern China.