Pohiva died at the Auckland City Hospital at about 9 a.m. after being medically evacuated to New Zealand a day earlier, political adviser Lopeti Senituli told The Associated Press. Prior to that, he had been hospitalized in Tonga for two weeks suffering from pneumonia before his condition turned critical, Senituli said.
"He will be remembered as the champion of democracy and being primarily responsible for the democratic reforms that were incorporated into the country's constitution in 2010," Senituli said.
Pohiva was also known for his fight against global warming. Archipelagos like Tonga, which is made up 171 islands and is home to 106,000 people, are particularly vulnerable to rising seas.
Pohiva was first elected prime minister in 2014 and won re-election three years later. His recent tenure was marked by bouts of ill health.
Pohiva spent more than three decades in political office after he was first elected to Tonga's parliament in 1987. In 2013, he became the first Pacific Islander to win the Defender of Democracy Award, presented by New York-based nonprofit Parliamentarians for Global Action.
"His political career has been marked by battles with the Tongan monarchy over democracy, transparency and corruption," the nonprofit wrote, noting that he was imprisoned in 1996 for contempt of parliament before the Supreme Court ordered he be released. On another occasion, he was charged with sedition.
"He was an immensely significant figure," said Graeme Smith, a research fellow at Australian National University. "As prime minister, he was very influential in the region and a really strong voice for Tonga. Regionally, and globally, he will be tremendously missed."
Smith said it may be too early to tell if Pohiva has created a permanent legacy of strong democracy in Tonga, because there continues to be pushback from vested interests including the royal family and the nobility.
For now, lawmaker Semisi Sika is Tonga's acting prime minister.
Australian Prime Minister Scott Morrison said he was "terribly saddened" to hear of Pohiva's death.
"He was a passionate advocate for his people, for his beloved Tonga & our Pacific family," Morrison wrote on Twitter.
New Zealand Prime Minister Jacinda Ardern said Pohiva would be remembered for his "lifelong commitment to championing democracy."
Before becoming a politician, Pohiva taught history and sociology at the University of the South Pacific. His wife Neomai Pohiva died last year. The couple had seven children.
Pohiva spoke at a 2006 pro-democracy rally in the capital Nuku'alofa shortly before rioters destroyed much of the downtown. After that, the country borrowed money from China to rebuild and now owes $108 million to China's Export-Import bank, equivalent to about 25% of GDP, a level of indebtedness that worries many observers.
Fijian Prime Minister Voreqe Bainimarama wrote on Twitter that Pohiva "inspired the world with raw emotion" last month at the Pacific Islands Forum in Tuvalu. Bainimarama said Pohiva attended despite his poor health because he recognized the urgency of climate change: "We must honor his legacy by continuing this fight."