Travellers have begun streaming into mainland China by air, land and sea as Beijing opened borders that have been all but shut since the start of the COVID-19 pandemic.

After three years, mainland China opened sea and land crossings with Hong Kong and ended a requirement for incoming travellers to quarantine on Sunday.

China's easing over the past month of one of the world's tightest COVID-19 regimes followed historic protests against a policy that included frequent testing, curbs on movement and mass lockdowns that heavily damaged the second-biggest economy.

Long queues formed at Hong Kong's international airport for flights to mainland cities including Beijing, Tianjin and Xiamen and some Hong Kong media outlets estimated that thousands of people were travelling across.

"I'm so happy, so happy, so excited. I haven't seen my parents for many years," said Hong Kong resident Teresa Chow as she and dozens of other travellers prepared to cross into mainland China from Hong Kong's Lok Ma Chau checkpoint early on Sunday.

"My parents are not in good health, and I couldn't go back to see them even when they had colon cancer, so I'm really happy to go back and see them now," she said, adding that she plans to head to her hometown in eastern China's Ningbo city.

Investors hope the reopening will eventually reinvigorate a $US17-trillion economy suffering its lowest growth in nearly half a century. But the abrupt policy reversal has triggered a massive wave of infections that is overwhelming some hospitals and causing business disruptions.

The border opening follows Saturday's start of "chun yun", the first 40-day period of Lunar New Year travel, which before the pandemic was the world's largest annual migration of people returning to their hometowns of taking holidays with family.

Some two billion people are expected to travel this season, nearly double last year's movement and recovering to 70 per cent of 2019 levels, the government says.

Many Chinese are also expected to start travelling abroad, a long-awaited shift for tourist spots in countries such as Thailand and Indonesia, though several governments - worried about China's COVID spike - are imposing curbs on travellers from the country.

Travel will not quickly return to pre-pandemic levels due to such factors as a dearth of international flights, analysts say.

China on Sunday also resumed issuing passports and travel visas for mainland residents, and ordinary visas and residence permits for foreigners.

Beijing has quotas on the number of people who can travel between Hong Kong and China each day.