HONG KONG/COLOMBO July 12 (Reuters) - Sri Lanka raised $1.5 billion in its first sale of dual-tranche eurobonds at a lower borrowing cost than initially expected, as yield-hungry global investors put in $6.6 billion in offers.

Asian sovereign bonds have rallied this year as investors look for better returns in a low-rate environment. That appetite has grown since Britain's vote to leave the European Union sent more money into safe-haven assets such as government bonds.

"It was luck. Had we gone last week, it (the yield) would have been much higher than this. The order book saw $6.6 billion and we got $1.5 billion in both bonds," Finance Minister Ravi Karunanayake told Reuters on Monday.

The government raised $500 million from a 5-1/2-year tranche and $1 billion from a 10-year tranche.

The 5-1/2-year bond priced at a yield of 5.75 percent and the 10-year bond at 6.825 percent, a source close to the deal told Reuters.

In early deals the new bonds due 2022 were trading at 101.25 cents on the dollar and those due 2026 were at 101.875, higher than the pricing level of 100, indicated by brokers.

Sri Lanka's sovereign debt has been one of the best performers in Asia this year.

The International Monetary Fund's (IMF) executive boards in June approved a three-year $1.5 billion loan to support Sri Lanka's economic reform agenda, after the South Asian nation was hit by a balance-of-payments crisis.

Over 200 accounts bought each bond, with the 5-1/2-year paper attracting 35 percent of funds from the United States, 37 percent from Europe and the rest from Asia, while 85 percent of the funds were from fund managers.

The 10-year bond attracted 62 percent of funds from the United States, 28 percent from Europe, and 10 percent from Asia with, 91 percent came from fund managers.

The final guidance for the 5-1/2-year tranche was 37.5 basis points (bps) lower than initial guidance in the area of 6.125 percent. For the 10-year bond, final guidance was 25 bps lower than the initial guidance of around 7.125 percent.

In comparison, bonds due in 2021 and 2025 were trading at a yield of 5.5 percent and 6.6 percent, respectively, before the new bonds priced.

A Singapore-based analyst said ahead of the pricing that the 10-year tranche looked fairly valued at current levels and the 5-1/2-year was attractive as long as it topped 6 percent.

"After Brexit, investors are looking for safe havens and I think dual tranche is an opportunity to get these investors attracted to our bond deals," Karunanayake earlier told Reuters.

Moody's assigned a provisional B1 rating to Sri Lanka's offering.

According to JACI benchmarks, dollar bonds issued by Sri Lanka produced total returns of 10.11 percent in the year to date, more than established issuers such as the Philippines.

By comparison, German debt with maturities out to 15 years is yielding below zero and Dutch 10-year government bond yields fell below zero for the first time on Monday.

Citigroup, Deutsche Bank, HSBC and Standard Chartered are joint bookrunners for the Sri Lankan issue.

Last October, Sri Lanka sold a $1.5 billion 10-year bond at 6.85 percent.