Tonga's prime minister Akilisi Pohiva died in an Auckland hospital after years of ill health, New Zealand officials said Thursday, hailing the 78-year-old as a democratic pioneer in the Pacific kingdom.
Pohiva was airlifted from Tonga on Wednesday to receive treatment for pneumonia, when his office issued a statement urging people to pray for him.
New Zealand Prime Minister Jacinda Ardern said he died on Thursday morning.
"Pohiva was an elder statesman for the Pacific and a great friend to New Zealand," she said.
"He will be remembered for his lifelong commitment to championing democracy."
Australian Prime Minister Scott Morrison said he was "terribly saddened" at Pohiva's death.
"He was a passionate advocate for his people, for his beloved Tonga and our Pacific family," Morrison tweeted.
Pohiva, a former history teacher, began his push for democracy in the mid-1970s, when Tonga was an absolute monarchy dominated by the royal family and nobles.
"For Tongan reform for democracy, no light burned brighter than this man," the New Zealand First party's Shane Jones told parliament in Wellington.
"Not only did he face charges of treason... he was harassed for his beliefs. He rose to be prime minister and brought a level of equity to the political culture of Tonga."
Pohiva was elected to parliament in 1987 and became prime minister in 2014 after reforms that gave ordinary voters more say in their government.
"During his political career we saw him transition from firebrand activist to an elder statesman of the blue Pacific, always staying true to his strong ideals of democracy and human rights," Pacific Islands Forum Secretary General Meg Taylor.
King Tupou VI effectively sacked Pohiva in 2017 by dissolving parliament and calling a snap election, but the democracy champion was so popular that he defied expectations and won a second term.
Despite his physical frailty, he remained active in office and last month attended the Pacific Islands Forum in Tuvalu, where he lobbied for urgent action on climate change.
Tuvalu Prime Minister Enele Sopoaga said Pohiva was so moved by a plea to the regional leaders from two Pacific youths to safeguard their future from global warming that he wept.
"The leader of Tonga actually shed tears" at a presentation from "two young warriors of climate change", Sopoaga said.
Fiji Prime Minister Frank Bainimarama said Pohiva's raw emotion at the meeting was inspirational.
"We must honour his legacy by continuing this fight," he said.
Officials said Tonga's parliament was suspended indefinitely after Pohiva's death and his deputy Semisi Sika had taken over as acting prime minister.
Funeral arrangements have yet to be announced.