Tongan Prime Minister Lord Siale'ataonga Tu'ivakano said in a brief radio address Monday that the king died at 3 p.m. Sunday during a visit to Hong Kong. He didn't give a cause of death, but said further details and funeral plans would be released when available.
The prime minister said the king's younger brother and heir to the throne, Crown Prince Tupouto'a Lavaka, was at the king's side when he died.
Paula Mau, a spokesman for the Tongan government, said the king had been traveling in Europe and elsewhere since late November and appeared healthy in photographs taken a few weeks ago when he met with the pope in the Vatican. Mau said the king underwent an operation in the U.S. last year.
Mau said the king was a bachelor who had a daughter out of wedlock — but that she didn't have a claim to the throne under Tonga's constitution, which recognizes only children born in wedlock as legitimate heirs.
Tupou reigned over the island nation of 106,000 since his father, King Taufa'ahau Tupou IV, died in 2006.
The father long resisted ceding any powers of Tonga's absolute monarchy during his four-decade rule. But after his death, rioters unhappy with the pace of reforms took to the streets and destroyed the center of the capital, Nuku'alofa.
Against that backdrop, the new king delayed his official coronation until 2008 while he put together the framework for sweeping political reforms. Three days before the coronation ceremony, Tupou announced he was ceding most of his executive powers to a democratically elected parliament. The king remains head of state, and some parliament seats are reserved for nobility.
While the parliament is now responsible for much of the day-to-day running of the country, the king retains the right to veto laws, decree martial law and dissolve the parliament.
"He believed that the monarchy was an instrument of change and can truly be seen as the architect of evolving democracy in Tonga. This will be his enduring legacy," Key said.
Tupou V did allow himself an elaborate coronation, a five-day affair that included roast pig feasts, tribal rites and British-style pomp. Its price tag of $2.5 million put a heavy strain on the impoverished nation.
He will be remembered by many for his throwback fashion choices — which included wearing, at times, a top hat and even a monocle. He studied at King's College in Auckland, New Zealand, and in Britain.
Pesi Fonua, publisher of the Tongan news website Matangi Tonga, said the king also advocated for technologies such as cellphones and the Internet, and made some enemies among conservative Tongans for his efforts to make the economy more market-driven and open to foreign competition and investment.
The prime minister declared that the royal family and entire nation was in mourning. Fonua said he ended his address with a Tongan expression meaning "The sun has set."