China stocks tumbled in afternoon trade on Wednesday, despite surprisingly positive official economic data, as a recent post-rout, government-triggered rebound appears to be running out of steam.

The CSI300 index of China's largest listed companies tumbled over 5 per cent at one point, but eased some losses to end the day down 3.5 per cent, at 3,966.76. The Shanghai Composite Index lost 3.0 per cent, to 3,805.70 points.

The slide highlights the ongoing difficulty Beijing faces as it seeks to restore confidence in its stock market without signalling investors that it is guaranteeing zero-risk free for all, which would simply reinflate a rally that even regulators said had become too frothy.

"Sentiment is still weak," said Du Changchun, analyst at Northeast Securities in Shanghai, adding that he believed most investors were selling off to cash in on a brief if sharp rally that pushed up indexes more than 10 percent last week.

The souring sentiment caused index futures to go negative across the board.

China CSI300 stock index futures for July fell more than 4 per cent, while the futures tracking small cap CSI500 index saw most contracts near their max 10 per cent daily downside limit.

The fall comes as a fresh batch of companies resumed trading on Wednesday, leaving only about 25 percent of shares on trading halts, down from over half during the rout.

Better-than-expected Chinese economic data on Wednesday failed to impress some investors. The economy grew an annual 7 per cent in the second quarter.

"Investors liquidated their positions as the GDP data failed to impress while domestic consumption showed no sign of improvement," said Steven Leung, a director at UOB Kay Hian in Hong Kong.

Infrastructure, transport and health care stocks also fell sharply.

But banking heavyweights rose, as well as energy giant PetroChina, fuelling speculations that government-backed investors are helping stabilise the market.

One internet message described China's stock market status quo of late this way: "Either a thousand shares hit limit up, or a thousand shares stay limit down, or a thousand shares are suspended of trading."

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