The Hong Kong Observatory issued the Standby Signal No. 1 at 8.40am on Wednesday as Severe Typhoon Podul comes within about 800 kilometres of Hong Kong.

Forecasters say the No.1 Signal will remain in force during the day and will assess the need to issue a higher signal on Wednesday evening to Thursday morning.

"Depending on the distance between Podul and the Pearl River Estuary, its intensity and the change in local winds, the Observatory will assess the need to issue the Strong Wind Signal, No. 3 tonight to tomorrow morning," a spokesman said in a statement on Wednesday.

"Due to the fast moving speed of Podul, according to the present forecast track, Podul will make landfall again over the vicinity of southern Fujian to eastern Guangdong overnight."

Podul is forecast to be closest to Hong Kong on Thursday morning.

The observatory added that the subsiding air of the weather system will bring very hot weather, with temperatures possibly rising to 35 degrees or above.