Hong Kong officials were Monday investigating a weekend ferry crash that injured 124, including five critically, amid fresh concern over the safety of its busy waterways.

Investigators still did not know what caused Sunday's accident, when a high-speed ferry from Macau to Hong Kong hit an "unidentified object" in the water.

"We feel saddened by the large number of people who were affected by the collision... the Marine Department will look into the cause of it," said the city's transport secretary Anthony Cheung.

Of the 163 passengers and 11 crew on board, a total of 124 aged between six and 83 were injured.

Hospital authorities said Monday most have been discharged, but 33 remained in hospital including five critically injured.

Ferry operator Shun Tak said water had seeped into the vessel's engine rooms and the rear wing was damaged after the collision.

A major rescue operation was mounted after the ferry lost power following the collision.

Media reports described scenes of chaos, and photos showed wounded people being stretchered onto ambulances at a pier in Hong Kong's Central financial district.

The South China Morning Post reported that a rubber tyre was retrieved afterwards near where the ferry crashed and investigators are trying to find out whether it had any role in the accident.

The city has one of the world's busiest ports, with hundreds of high-speed ferries, cargo ships, fishing boats and yachts criss-crossing it daily.

But fears over its maritime safety have grown following a spate of accidents in recent years, including a crash in 2012 that killed 39 people when a high-speed ferry collided with a pleasure boat near Lamma Island.

A subsequent inquiry found a "litany of errors" contributed to the accident, the city's worst maritime disaster in over 40 years.

In 2013 another high-speed ferry crashed into an unidentified object, injuring 87 and raising concerns over the problem of rubbish-strewn waterways.

In June 2014 more than 50 people were injured when a Macau-bound ferry crashed into a seawall.