Giant hailstones smashed windows at a Hong Kong shopping mall as torrential rainstorms drenched the Sevens rugby tournament and delayed dozens of flights.

Authorities issued the highest black rainstorm warning late on Sunday when rainfall exceeded 80 millimetres an hour in parts of the territory.

The Hong Kong Observatory said it had recorded 8,703 lightning strikes, with more than 3,500 in just one hour, according to the AFP news agency.

The rainstorm caused flooding, shutting down several roads and causing delays to train services, while more than 100 flights had to be delayed due to "adverse weather", the authorities said.

In an upscale shopping mall in Kowloon Tong district, torrents of water poured through the roof after glass panes were smashed by giant hailstones, local media reported.

Television footage showed umbrella-carrying shoppers wading through the drenched mall.

At the Hong Kong Sevens, New Zealand performed their traditional haka during a tropical downpour before defeating England 26-7 in a rain-drenched cup final.

Elsewhere, a stack of about 20 containers collapsed at the docks, crushing a truck, local broadcaster RTHK reported.

The Observatory said more rain was expected in the next few days, with "squally thunderstorms" at times.

It said the stormy weather was due to a trough of low pressure affecting coastal areas of neighbouring Guangdong province in southern China.

Source: AFP