Investors from the Chinese mainland for the first time on Wednesday used the entire 10.5 billion yuan ($1.69 billion) daily investment quota for buying Hong Kong stocks under the Shanghai-Hong Kong Stock Connect program.

The buying boosted the Hang Seng Index, which soared more than 3.7 percent. The Hang Seng China Enterprises Index of Hong Kong-listed mainland companies was up 5.7 percent for the day.

The milestone follows signs of rising interest in Hong Kong stocks from mainland investors, after months in which the southbound leg of the stock program was largely unused.

Chen Zhizhong, a Shenzhen-based analyst at China Merchants Securities Co Ltd, said that the recent three-day weekend was full of discussion in the analyst community about whether the time was right to move into Hong Kong.

"The party has begun, and you can feel the excitement today," Chen said. "It's hard to say when the music will stop."

Over the past year, China's CSI300 index has soared more than 90 percent, while the Hong Kong China Enterprises Index rose just 28 percent.

This divergence caused the index measuring the price difference for dual-listed companies, commonly known as the "A-share premium", to shoot up to a multi-year high of 135 on March 24, implying that Shanghai shares were trading at a one-third premium to shares in the same company listed in Hong Kong.

That index plummeted as southbound flows increased in recent days, and it now hovers near 124.

Analysts said that one reason for the increased southbound flows was that Chinese regulators last week allowed mutual funds to buy Hong Kong shares under Stock Connect.