HONG KONG, June 21 (Reuters) - Legislators in Macau approved a landmark casino bill on Tuesday, the biggest reforms in more than two decades, ahead of an expected casino licence bidding process in the world's biggest gambling hub.

The changes - marking the first major reforms since casinos were first liberalised in 2002 - mean far greater oversight and control by the government into casino operations in Macau.

The vote came as the Chinese special administrative region carries out mass COVID-19 testing of its more than 600,000 population after dozens of locally transmitted cases were found in recent days.

A hotel and casino resort on Macau's main peninsula was locked down by authorities on Tuesday due to a coronavirus infection, with some 700 people forced to quarantine.

Macau's legislature released a final draft of a highly anticipated gaming bill last week, fixing casino tax to 40% from 39% currently and giving officials far greater oversight into casino operations

The 55-page document, released in Portuguese and Chinese on the legislature's website, comes as the government is due to formally sign a six-month licence extension for casino operators this week.

The moves are expected ahead of casino license expirations due this month.

The extension, to Dec. 31, allows more time for a highly anticipated rebidding process. Macau is the only place in China where gambling in casinos is legal.

The final version is similar to an initial draft first circulated in January but clarifies that casino tax on gross revenues will increase to 40% although the chief executive has discretion to reduce up to 5% if operators succeed in attracting non-mainland Chinese gamblers.

Casino operators must have 5 billion patacas ($618.43 million) in cash at all times during the 10-year license period.

Macau's casino operators - Wynn Macau, Sands China , MGM China, Galaxy Entertainment and Melco Resorts - all have sufficient liquidity with only SJM Holdings needing to beef up liquidity, analysts said.

(Reporting by Farah Master Editing by Bernadette Baum)