Hong Kong marked the completion of a third runway at its airport on Tuesday at a time when the once thriving international travel hub remains cut off from most of the world.

City leader Carrie Lam presided over a topping-off ceremony for the 3.9-kilometre runway, which took five years to construct on reclaimed land.

Thanks to its location and comparatively relaxed entry requirements, Hong Kong has long hosted one of the world's busiest international airports.

But it faces increasing competition from regional rival Singapore as well as rapidly expanding airports in mainland China.

The city's reputation as a travel hub was also dented by months of political unrest in 2019, which at one point paralysed the airport, and China's subsequent crackdown on dissent.

The business hub currently remains inaccessible to most people during the coronavirus pandemic because it maintains some of the world's strictest quarantine measures.

Almost all arrivals must complete one to three weeks of mandatory hotel quarantine, a move that has kept the coronavirus at bay but hammered the travel industry and left the city isolated.

A view of the tarmac on the completed third runway at Chek Lap Kok airport in Hong Kong
A view of the tarmac on the completed third runway at Chek Lap Kok airport in Hong Kong AFP / Peter PARKS

The construction of new runways often faces strong opposition from environmental groups in western nations but Hong Kong's airport expansion saw little protest.

Before the coronavirus, the two runways were already operating well beyond their capacity of receiving and sending 420,000 flights per year.

The third runway is expected to start operations sometime in 2022.

It is unclear whether Hong Kong will have loosened its travel restrictions by then.

Despite ample supplies, the city has one of the worst Covid-19 vaccination rates in the industrialised world and the government has given no details on when it might move towards living with the coronavirus.

International businesses have grown increasingly frustrated, with the European Chamber of Commerce recently warning that residents were "indefinitely trapped".

But last week Lam doubled down on her zero-Covid policies and said opening travel with the Chinese mainland was more important than doing the same for the rest of the world.