Hong Kong police say they were investigating a fifth acid attack in a year after six people were injured by a bottle of corrosive liquid in a bustling shopping district.

The plastic bottle was hurled in Causeway Bay on Saturday, one of the city's busiest areas, around 10 pm (1am today, Australian time), a police spokeswoman told AFP.

Five women and a man suffered burns and were sent to hospital for treatment. One of the women's face and chest were badly injured, with burns covering nearly 20 percent of her body.

Causeway Bay was packed on Saturday night with people celebrating the Hong Kong football team's shock victory over Japan in the East Asian Games final at a nearby stadium.

Detectives are investigating but "it remains unclear whether the case is linked to the previous acid attacks", the spokeswoman said.