King George Tupou V of the South Pacific nation of Tonga has died at the age of 63, reports say.
He died at a hospital in Hong Kong with his brother, Crown Prince Tupouto'a Lavaka, at his side, Matangi Tonga Online and NZ media said.
Television New Zealand said the Tongan government had confirmed his death.
Tonga is the last Polynesian monarchy, although the king was instrumental in bringing about democratic reform. He had been king since 2006.
People across the 170-island archipelago were able to vote for their first popularly-elected parliament in November 2010, ending 165 years of feudal rule.
King Tupou said on the eve of the vote he was granting his executive powers to the cabinet and parliament, adding that "in future the sovereign shall act only on the advice of his prime minister".
The cause of King Tupou's death on Sunday was not immediately known.
Many Tongans paid tribute to the monarch, after the news of his death quickly spread on the internet.
"I am saddened by the news, our beloved King of Tonga has passed away... our country has gone through major losses in the past couple years," Tessi Leila Tolutau was quoted as saying by the New Zealand Herald newspaper.
In September 2006, King Tupou succeeded his father King Taufa'ahau Tupou IV, who died after a long illness.
But deadly riots two months later, by people angry at the slow pace of reform, delayed his coronation until August 2008.
The Oxford-educated monarch was a bachelor. He named his brother
King Tupou was known for his liking of wearing military uniforms and for driving around in a London taxi.