The prime minister declared that the royal family and entire nation was in mourning, ending his address with a Tongan expression meaning "The sun has set," according to Fonua.
The king had a liver transplant last year and suffered other health problems, according to Tongan media reports.
Tupou had reigned over the island nation of 106,000 since Sept. 2006, after the death of his father, King Taufa'ahau Tupou IV.
Tupou, who studied at King's College in Auckland, New Zealand, and in Britain, is credited with championing a more open system of government, advocating technological improvements and introducing a more open economy in the kingdom.
He will be remembered by many for his throwback fashion choices which included wearing, at times, a top hat and even a monocle.
New Zealand's Prime Minister John Key released a statement Monday saying that his thoughts were with the people of Tonga.
"I would like to acknowledge the very valuable contribution the king has made in steering Tonga toward democracy and hope this work will continue," Key said. "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."
Fonua said the king gave up most of his executive powers when he came to the throne, accepting the need for a more democratic system. Fonua said the king also championed technologies such as mobile phones and the Internet, and made some enemies among conservative Tongans for his efforts to make the economy more market-driven.
Many Tongans live in Utah
One of every four Tongans living in the United States resides in Utah, according to a Salt Lake Tribune analysis of 2010 U.S. census data. Utah ranks second among the states for total population of Tongans, behind only California but ahead of even Hawaii. Utah's Tongan population is an eighth as large as the population of the Kingdom of Tonga, which equates to a scenario in which one of every nine Tongans leaves home to live here. Salt Lake City has more Tongans than all but the three largest cities among the 176 islands of Tonga.