By Viparat Jantraprap and Orathai Sriring

BANGKOK, March 5 (Reuters) - Overseas investors who helped make the Thai stock market the best performing in Southeast Asia in 2012 are now pulling money from a market that is more expensive than its Asian peers.

The Thai stock market has risen another 11 percent this year, following a stellar 36 percent rally in 2012.

Yet, Thomson Reuters data shows foreign investors sold almost $600 million of stocks in February after buying $500 million in January and $2.5 billion in 2012, when they rebuilt positions after serious floods put them off in late 2011.

"Valuations of Thai stocks are clearly stretched and the market needs positive triggers such as earnings to attract investors again," said strategist Chaiyaporn Nompitakcharoen at Bualuang Securities.

The economic backdrop has, if anything, improved since the end of last year, with inflation under control, interest rates set to remain low for months and the Bank of Thailand poised to raise its GDP growth forecast for 2013 from 4.9 percent.

The government is also about to finalise plans for a huge, 2.2 trillion baht ($74 billion) infrastructure programme, which would be "a major market driver", said Thanomsri Fongarunrung, an economist at Phatra Securities.

But according to Thomson Reuters Starmine, Thai stocks are trading at 20.4 times price to earnings, higher than Indonesia's 18.6, Malaysia's 15.3, Singapore's 14.1 and Vietnam's 14.0. The Philippines is higher at 21.1 times.

In Southeast Asia, Malaysia, Indonesia and the Philippines reported inflows in the first two months of this year. Flows into Indonesia of $1.76 billion in 2013 are higher than the $1.64 billion it attracted in the whole of 2012.

Thai stock exchange data shows net short positions on index futures in February of more than 6,000 contracts, double the number in December.

In contrast to foreigners, local funds continue to buy the market. Domestic institutions and retail investors make up around 80 percent of trading by value.

Thomson Reuters Lipper data showed funds in Luxembourg and the United States were still buying, with net inflows into Thai offshore equity funds of $377 million in January and $258 million in February. Onshore funds showed a foreign outflow of $55 million in January but an inflow of $353 million in February.