SAO PAULO, Sept 15 (Reuters) - Brazilian infrastructure concession firm Ecorodovias on Thursday won an auction to operate highways in central Sao Paulo state, offering a vast premium over the minimum bid.

Ecorodovias shares fell sharply after the auction results to end down nearly 12% at the market close, compared with a drop of just 0.5% in the Ibovespa index.

The company won the concession for highways linking affluent cities in the interior of Sao Paulo state, such as Sao Jose do Rio Preto, Araraquara, Sao Carlos and Barretos, with a fixed concession value of 1.236 billion reais ($236 million).

Ecorodovias' offer was more than 160 times the minimum bid of 7.6 million reais ($1.45 million) and was 64% higher than a rival 753.8 million reais bid by transportation company CCR .

The 30-year concession comprises about 600 km (373 miles) of highways and is expected to include 10 billion reais in upgrade projects and another 3.9 billion reais in operations, according to Sao Paulo's government.

"The northwest lot will generate EBITDA gains from the beginning of the concession and extends the duration of the portfolio," Ecorodovias said in a securities filing, adding that its offer "clearly demonstrates the company's commitment to growth with sustainable returns and financial discipline."

($1 = 5.2470 reais) (Reporting by Andre Romani; Editing by Leslie Adler)