* Hang Seng pulls back from 8-mth high, seen oversbought
* Shanghai Composite down 0.6 percent, financials weigh
* Chalco shares surge on rare earth investment
* China Unicom down after convertible bond issue
* Ningbo Port IPO ends below listing price in Shanghai (Recasts with closing of two bourses)
By Vikram S.Subhedar and Farah Master
HONG KONG/SHANGHAI, Sept 28 (Reuters) - Hong Kong and Shanghai stocks fell on Tuesday as a mild selloff in large caps drove the Hang Seng down and weakness in financials and poor debut of Ningbo Port weakened China's key stock index.
The benchmark Hang Seng, which is poised to post its best monthly gain since July 2009, fell 1 percent to 22,109.95 led by local property developers and index heavyweights.
A four-week rally, supported by recovering turnover, had taken the index well into overbought territory although Tuesday's drop pushed its relative strength index back below the threshold 70 level.
The next resistance for the index is at its 2010 peak, around 23,100, the level it touched in January.
However, a flurry of share and convertible bond placements in Hong Kong and a spate of initial public offerings could dampen further gains, analysts at Julius Baer said in a note.
China Unicom shares, the top loser amongst large caps, fell 4.3 percent after the mobile provider said it had sold $1.8 billion in convertible bonds, a move likely to dilute value for existing shareholders.
"Investors are probably concerned about why Unicom is issuing convertible bonds rather than straightforward debt," said Paul Wuh, an analyst at Samsung Securities in Hong Kong.
Ruinian International Ltd slumped 8.3 percent after the company said Turrence Ltd, Templeton Private Equity and other shareholders were selling 100 million shares at a discount to the Monday closing price.
Activity on the primary market in Hong Kong has also picked up considerably with about 20 companies looking to raise more than $21 billion
having listed this month or planning a listing in October to take advantage of improved market conditions.
That includes the initial public offering of AIA Group, the Asian unit of AIG, which began pre-marketing for its offering which could raise as much as $15 billion.
SHANGHAI SLIPS
The Shanghai Composite Index fell 0.6 percent to 2,620.75. Analysts said the 60-day moving average at 2,560 should provide strong support ahead of the week-long National Day holiday starting on Friday.
The sluggish market took its toll on shares of Ningbo Port which closed 3.5 percent below their listing price. The company raised $1.1 billion in the country's sixth-biggest IPO this year..
Analysts said weak overseas markets and concerns about global economic growth weighed on financials and resources plays.
"Financials and banks are heavy weights on the index so at a time of uncertainty when they are under pressure, the broader index is unable to gain," Nanjing Securities' Wen Lijun said.
"Market confidence is still lacking ahead of our national day holiday, where we will face more uncertainty," said Wen.
Jiangxi Copper fell 6.7 percent, Coal miner Shan Xi Guo Yang New Energy slipped 3.1 percent, Lizhou Iron & Steel fell 4.2 percent.
Financials sagged with China CITIC Bank Corp down 2 percent, while Industrial Bank fell 2.6 percent. Agricultural Bank of China slid 0.8 percent.
Bucking the broader trend, Chalco jumped by its 10 percent daily limit after its parent said it would take a majority stake in a rare earths company. ($1=6.691 Yuan)