Rescue teams in Hong Kong Friday stepped up the search for six crew members missing at sea after two boats collided in fog, as officials investigated the cause of the latest incident to hit the city's busy waterways.

The incident comes after a ferry collision claimed 39 lives in October in the city's worst sea disaster in decades, and also follows a crash between a passenger ferry and a barge earlier this month, in which more than 30 people were injured.

The 11 crew from a 96-metre-long (315 foot) boat carrying construction waste were thrown into the water after it collided with another vessel and sank off Stanley, on the southeast of Hong Kong Island Thursday evening.

Five were rescued but six remained missing after an overnight search hampered by low visibility.

"The rescuers have searched for them overnight but we are still looking for the six missing," a spokeswoman from the marine department told AFP.

"The direction of the probe is to investigate the cause of the incident, especially to see whether anyone took measures to prevent the collision at sea."

Additional police divers and five more rescue boats were dispatched to the area Friday, to assist the four vessels already at the scene, she said. But a search helicopter could not be deployed because of continuing poor visibility.

A police spokeswoman said officers had taken statements from seven crew members from the two boats.

The marine department said visibility at the time of the crash was measured at 0.5 nautical miles (0.93 kilometres).

TV footage showed two of the five rescued crew members on stretchers, apparently conscious, as they were taken to hospital.

The city's worst maritime catastrophe in 40 years last October saw a high-speed passenger ferry collide with a pleasure boat carrying around 120 people to watch national day fireworks.

The captains of the boats involved were last week each charged with 39 counts of manslaughter and face life in prison if convicted.

Fatal accidents are rare in Hong Kong despite its crowded waters, which often see high-speed hydrofoils vying for space with tourist junks, luxury yachts and a century-old public ferry system.