(Reuters) -Australian bank Westpac declared a special dividend on Monday and raised its share repurchase program by A$1 billion ($661 million) citing a strong balance sheet even as its first-half profit fell by 16% on intense competition and high costs. Traditionally beneficiaries of rising interest rates, the country's so-called Big Four lenders have spent the past year sacrificing margins to write new home loans and paying more to depositors, narrowing their closely watched "net interest margin". Westpac's net interest margin slipped to 1.89%, down 7 basis points from a year ago, while net interest income remained largely flat at A$9.13 billion.