Hong Kong opened a temporary field hospital with 500 beds on Saturday to house stable COVID-19 patients as the city battles a new wave of virus infections.

The financial hub had been a poster child for tackling the coronavirus, with local transmissions all but ended by early summer. But since July the pathogen has returned, with some cases brought in by the tens of thousands of people who were exempted from a mandatory quarantine imposed on most arrivals.

The opening of the hospital comes a day after city leader Carrie Lam announced the postponement of upcoming legislative elections for a year, saying the decision was purely to protect public health and had "nothing to do with politics".

More than 2,000 new infections have been detected since the start of July -- 60 percent of the total since the virus first hit the city in late January. The new makeshift hospital set up at the city's AsiaWorld-Expo exhibition centre near the airport, will take in COVID-19 patients aged from 18 to 60.

"Our goal is to triage the patients from the community and provide them with appropriate treatment and hospitalisation arrangement," doctor Larry Lee from the Hospital Authority told media, adding that around 20 to 30 patients are expected to be admitted on the first day.

Lee said that arriving patients will go through X-rays as well as blood, saliva and heart tests. If found unstable, patients will be sent to hospitals while stable ones will get treatment and be put under medical observation at the facility.

The city recorded 125 new cases on Saturday, the 11th day in a row above 100, with total infection numbers rising to 3,396. The death toll has risen from seven to 31 since July.

China has offered to send mainland testing staff to the city to carry out "large scale nucleic acid testing". The National Health Commission of China said in a statement it had set up a team comprised of 60 clinical laboratory staff and an "advance unit" will arrive in the city on Sunday.