According to data from the Ministry of Commerce, the National Bureau of Statistics and the State Administration of Foreign Exchange last September, the mainland's outbound direct investment grew at an annual average rate of nearly 40 percent between 1995 and 2015.
The country's ODI flow reached an historical high of US$145.7 billion in 2015.
The value of assets held by mainland companies in offshore locations climbed to over US$4 trillion in the same period, according to the data.
Hong Kong has always been a conduit for channeling ODI from the mainland, as well as attracting foreign direct investment into the mainland over the last two decades.
Leung and other panelists told the forum that Hong Kong could contribute to promoting the Belt and Road Initiative in various aspects and this could endow further growth engines for the city.
The business forum on June 14 focused on the mainland's outward investment trends and the role of Hong Kong.
"Hong Kong can be the first destination for mainland enterprises going out because of the city's free flow of capital, access to market information, robust financial regulation, well-connected telecommunications and infrastructure networks and familiarity with international markets," said Leung, who is also a vice-chairman of the National Committee of the Chinese People's Political Consultative Conference.
The Belt and Road Initiative is a vision that connects China with 65 countries and regions with 4.4 billion people that generate about 40 percent of the global gross domestic product, according to the World Bank.
"Hong Kong's role in the B&R should be more diversified and geared to high-end contributions," said Li Wei, director of the Development Research Center of the State Council.
"For example, the city's services sector can play the role of business arbitration as well as project counseling, helping mainland companies to mitigate investment and financing risks involving B&R projects," Li added.
Antony Leung Kam-chung, chairman and chief executive officer of Nan Fung Group and former financial secretary, said Hong Kong's role in channeling investment for B&R infrastructure projects is important.
"Of the US$8 trillion B&R infrastructure investment required by the Asia Infrastructure Investment Bank, US$7 trillion is still not funded," Leung said.
"Hong Kong can leverage its niches as a financing platform to attract long-term capital into those B&R infrastructure projects."
Tian Guoli, chairman of Bank of China and BOC Hong Kong (Holdings), said the lender had issued B&R bonds in 2015 and 2017 respectively that raised US$7 billion, illustrating B&R projects were becoming a popular investment choice for global investors.
Wang Zhenmin, the legal chief for the Liaison Office of the Central People's Government in the Hong Kong Special Administrative Region, said Hong Kong could provide the legal risk monitoring service for mainland companies when they are venturing overseas.