Two of Hong Kong's best-performing stocks plunged more than 40 percent Thursday, a day after a mysterious crash of almost 50 percent in Chinese solar firm Hanergy that saw almost $20 billion wiped off its market value.

Goldin Financial sank 43.34 percent to HK$17.48 and Goldin Properties crashed 40.91 percent to HK$14.36, after soaring more than 300 percent since the start of January, according to Bloomberg News.

The drop slashed the firms' combined market value by more than $20 billion.

The companies, which have interests ranging from property development in Hong Kong and China to vineyards in California and France are owned by Chinese tycoon Pan Sutong.

The dramatic sell-off came after a 47 percent dive in Beijing-based solar energy firm Hanergy Thin Film Power (HTF).

Trading in the firm was suspended after 24 minutes, but not before $19 billion was struck off the firm's value. The company said it would make an announcement containing "insider information" in the wake of the suspension, although it has not yet done so.

HTF had surged more than sixfold in the past year, making it the world's largest solar power company by market value, but prompting questions over its valuation and revenue sources.

When contacted by AFP, a spokeswoman for Hong Kong's Securities and Futures Commission (SFC), the city's market watchdog, refused to say whether the company was under investigation. "We cannot comment on individual cases," the SFC spokeswoman said.

A public relations officer for Hanergy could not be reached for a comment on Thursday.

Hong Kong-based analyst Castor Pang, head of research at Core Pacific-Yamaichi International, said the collapse of three stocks within two days was "not a good sign" for the Hong Kong market.

"The substantial surge and crazy drop of the share price had no reason," he told AFP.

"If all (stocks) are in bubbles, it prevents the functioning of the market. If the situation continues, real investors may go away," he said.

The city's financial secretary John Tsang recently urged investors to be wary of market fluctuations.

"We can foresee after the market has accumulated some growth, it's just a matter of time for an adjustment to happen... I call upon investors to measure against the trend and exercise caution," he said Monday in a speech at a financial forum.