The Strong Wind Signal No. 3 took effect at 9.40pm on Monday and the observatory said it will issue the No. 8 signal at 2.20pm on Tuesday, as Super Typhoon Ragasa edges closer to Hong Kong.

Forecasters said they will assess the need of issuing higher signals "in the latter part of [Tuesday] night to at first of Wednesday", depending on the distance between Ragasa and Hong Kong as well as the change in local wind conditions.

At 10pm, the storm was about 680 kilometres east-southeast of the SAR and is forecast to move west or west-northwest at about 22 kilometres per hour across the northern part of the South China Sea.

The observatory said according to the present forecast, Ragasa will maintain super typhoon intensity and edge towards the coast of Guangdong, and will be closest to the vicinity of the Pearl River Estuary on Wednesday morning.

The super typhoon has sustained winds of about 230 kilometres per hour near its centre.

The forecaster warned of "persistently adverse" weather on Wednesday, with gale to storm force winds prevailing and reaching hurricane force offshore and on high ground, along with frequent heavy squally showers and thunderstorms.

Water levels are expected to rise by about 2 metres over the coastal areas on Wednesday morning.

"The maximum water level can generally reach around 3.5 to 4 metres above chart datum, and the water level at Tolo Harbour may even reach 4 to 5 metres above chart datum," the observatory said.

Members of the public are urged to stay away from the shoreline and not to engage in water sports.