By Prudence Ho 

Chinese and Indian companies and their private-equity owners are taking advantage of booming stock markets to sell shares, lifting block trades in Asia to record highs.

These trades, in which companies or shareholders sell a large block of shares to investors outside of the open market, have reached US$29 billion year to date in the Asia-Pacific region, according to data from Dealogic. The activity coincides with a surge in Asian stocks--the MSCI Asia Pacific index is up 11% this year. Markets in Shanghai and Shenzhen have been the region's best performers, followed by Hong Kong, whose market is dominated by Chinese shares. Indian shares, while flat this year, are up 29% in the past 12 months.

Over the first four months last year, Asian stock markets were falling, and block trades were less than half this year's--at US$12.6 billion. The last time block trades had reached record levels (US$23 billion) for the same January-to-April period was in 2013, when the U.S stock market was rising and Asian stocks gained on the back of that advance. This year, the Dow Jones Industrial Average is up 1.5% as if midday Monday in New York, although it continues to break records.

"Equity markets overall have been going up in Asia, especially the Hong Kong and [Chinese] A share-markets, triggering existing shareholders to consider offloading their shares," said Hemant Sabherwal, Asia director of equity capital markets at Deutsche Bank AG.

Most of the selling has been of existing shares, as indicated by the top four block trades this year. The biggest such trade was in January, when the Indian government disposed of US$3.7 billion worth of shares in state miner Coal India, followed by U.S. oil giant Chevron Corp.'s exit from its US$3.7 billion stake in March in Caltex Australia Ltd., which runs an oil-refining business in that country. Australian stocks are up 7.7% this year, while Caltex Australia shares had risen around 10% before the sale.

The third-biggest block trade this year was by Japanese drug marker Daiichi Sankyo, which disposed of its US$3.2 billion stake in Indian drugmaker Sun Pharmaceutical Industries. Ranking fourth was a US$2.1 billion sale by Hang Seng Bank, a Hong Kong- and China-focused lender owned by HSBC Holdings, of its stake in Shanghai-listed Industrial Bank Co.

Mr. Sabherwal said he expects that, at some point this year, companies will take advantage of rising share prices to sell new stock. That, say fund managers, could be the point at which investors become less comfortable buying shares, especially in Hong Kong, where volumes have surged.

"Some investors, especially mainland Chinese investors, see Hong Kong stocks at a bargain, and so are piling in to buy" the shares of Chinese companies that trade at a premium on mainland bourses, said Alex Wong, head of asset management at Ample Capital in Hong Kong. But once more and more Chinese companies sell new stock, investors may be more wary of buying shares, he said.

For now, block trades keep booming, as regional share prices climb. The Shanghai Composite Index is up 39% this year, while Hong Kong's Hang Seng Index has gained 19%.

Last month, Citigroup raised its forecast for the Hang Seng to a record high of 32000 this year. The Hang Seng's closing level of 28123.82 Monday translates to about 12.5 times 2015 earnings, and in previous bull markets valuations have reached 17 to 29 times forecast earnings.

Bankers say Chinese companies catering to investor appetite for stocks include banks, which benefit from a looser monetary policy implemented recently, and insurers, which have chalked up strong gains from their investments in the domestic stock market.

Other shareholders that have jumped on the block-trade bandwagon this year have been private-equity firms cashing in on the market rally and disposing of long-held stock. In April, for example, Chinese private-equity firm Hony Capital, which bought U.K.-based pizza chain Pizza Express last year, raised US$1.26 billion by selling shares in CSPC Pharmaceutical Group Ltd., a Chinese maker of vitamins and generic drugs. Hony bought into the company in 2007.

Meanwhile, initial public offerings in the region this year have totaled US$21.7 billion, largely from a spate of Chinese brokerages that have listed in Hong Kong and Shanghai.

Write to Prudence Ho at prudence.ho@wsj.com

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