The Chinese finance hub saw scores of fallen trees with flooding in at least two seaside neighbourhoods, while buildings across the city rattled and swayed in the strong winds.

Ragasa is expected to be closest to Hong Kong on Wednesday morning, skirting just 100 kilometres (62 miles) south of the city, according to the Hong Kong Observatory, which issued the highest level of typhoon warning overnight.

"There will be overtopping waves over the shoreline, which will be particularly significant along the eastern and southern coasts," the observatory said.

"Under the influence of significant storm surge, around two to three metres of rise in water level are expected over the coast of Hong Kong today."

In Taiwan, the bursting of a barrier lake killed at least 14 people after the island was pounded by Ragasa, with 18 reported injured, according to regional officials.

The super typhoon also killed at least two people while ripping through the northern Philippines.

China's Ministry of Emergency Management said the typhoon is expected to make landfall along Guangdong's coast between Zhuhai and Zhanjiang from midday to late Wednesday.

Authorities across mainland China have ordered businesses and schools to shut down in at least 10 cities across the nation's south, affecting tens of millions of people.

- Neighbourhood flooded -

Multiple districts in Hong Kong saw instances of flooding, according to images circulated on social media and verified by AFP.

Floodwaters rushed into the seaside Heng Fa Chuen residential estate and covered its interior courtyards, a video clip showed.

An AFP reporter saw cars driving through flooded roads in that area in the early morning.

Strong winds ripped off the top of a pedestrian footbridge, while a large tree toppled by the winds blocked a road in the eastern part of Hong Kong Island.

Rail operator MTR said train services on open sections were suspended, with limited service available on the underground sections.

Authorities said more than 730 people sought refuge at the 48 temporary shelters across Hong Kong.

The city's airport operator said "only a limited number of cargo flights" were scheduled for Wednesday, with its website listing hundreds of cancelled passenger flights.

A five-year-old boy and his mother fell into the sea on Tuesday afternoon while they were watching the waves in the Chai Wan district, according to police. Both were in critical condition after they were rushed to hospital.

The boy's 40-year-old father, who reportedly jumped into the water to save his family, was also hospitalised.