TEHRAN, Young Journalists Club(YJC)_Pohiva, 78, had been diagnosed earlier this year with a liver complication and was admitted to a hospital in Tonga two weeks ago for pneumonia, the prime minister's office said in a statement on Wednesday.
Medical experts decided he should be medically evacuated to New Zealand, it said.
Parliament has been deferred indefinitely, Tongan media said.
Known as a champion for democracy, Pohiva's political career was marked with constant battles against the monarchy of the South Pacific nation. He was charged with sedition in wake of pro-democracy riots in the capital Nuku'alofa in 2006.
"Terribly saddened to hear of the passing of Tongan Prime Minister, the Hon. 'Akilisi Pohiva. He was a passionate advocate for his people, for his beloved Tonga & our Pacific family," Australian Prime Minister Scott Morrison said in a Twitter post.
Pohiva, Tonga's longest-serving member of parliament since he was first elected in 1987, was outspoken about climate change and called for world leaders to help island nations, some of which are battling rising sea levels.
Pohiva also raised concerns about the mounting Chinese debt in the South Pacific.
Tonga is heavily indebted to China, with more than 60% of its external debt owed to Beijing through bilateral loans, according to the country's 2019-20 budget.
The small Pacific nation received a reprieve from China last year on the timing of debt repayments after it signed up to China's Belt and Road initiative.