A cause of death has not been released by the government.
"Tonga has just woken up to the sad news of the passing of King George Tupou V," said Paula Ma'u, an official in the Ministry of Information and Communication. "We are all in mourning."
Ma'u said Crown Prince Tupouto'a Lavaka, heir to the throne, was with the king "just before he passed away."
Pesi Fonua, editor of Matangi Tonga online news website, said the Oxford-educated king was known for his world travel and spending habits, but was also respected for his efforts to bring democratic reforms and modernize Tongo.
"This king brought in things like mobile phones and other advances that we couldn't dream would come to Tonga, and his push for democracy made a big impact on the people here," Fonua said.
However, he said, Tongans had "mixed feelings here in the community" about the king.
"He lived a bachelor life, always a single man, and he traveled a lot around the world and spent money. He brought us in some ways forward into the 21st century, but some people have different -- not as good -- views on him."
Australian Prime Minister Julia Gillard expressed her condolences in a statement Monday, crediting George V with guiding "his country through a critical process of constitutional change towards the establishment of a constitutional monarchy."
Tonga, with a population of 106,000, is the only monarchy that remains in the Pacific. George V ascended to the throne after the death of his father in September 2006 and quickly promised to speed up government reforms demanded by the people.
"Let us rebuild a new capital and a new Tonga," George V said in 2006, a week after a pro-democracy rally in the capital city of Nuku'alofa led to riots that left eight people dead and the central business district in ruins.
In 2008, he announced he was giving up most of the near-absolute power that his family held for centuries and allowing the prime minister to guide the day-to-day governmental affairs.
Tonga is an archipelago of 171 islands directly south of Western Samoa. Less than a third of the islands are inhabited. The present dynasty was founded in 1845 after the Tongan islands were first united.
