SEOUL, Dec 31 (Reuters) - South Korea's foreign exchange authorities sold a net $1.745 billion in currency market intervention transactions in the third quarter, more than double the amount sold in the second quarter, central bank data showed on Wednesday.

It was the fourth consecutive quarter authorities sold dollars to curb the won's decline against the dollar. The won was down about 3.7% during the three-month period.

(Reporting by Cynthia Kim; Editing by Tom Hogue)