Mr Pohiva, 78, had been hospitalised for two weeks in the Tongan capital of Nuku'alofa with pneumonia.

He was urgently transferred to New Zealand on Wednesday night, but passed away on Thursday morning.

Mr Pohiva, who was first elected in 1987, is one of Tonga's major political figures of this century.

He was first elected as prime minister of the tiny Polynesian state in 2014 after decades of campaigning for democratic representation and reforms.

His advocacy led him to be arrested and thrown in jail, along with two journalists, for contempt of parliament in 1996.

His life included many clashes with the Crown in the fiercely monarchical state, as he argued for a relinquishing of powers from the King.

That came in 2008, when King George Tupou V gave over many responsibilities to the parliament and the government, clearing the way for Mr Pohiva's eventual prime ministership.

The Tongan Parliament has confirmed to AAP that it is on indefinite suspension in the wake of Mr Pohiva's death.

Australian prime minister Scott Morrison Tweeted his condolences.

"He was a passionate advocate for his people, for his beloved Tonga & our Pacific family," Mr Morrison wrote.

New Zealand prime minister Jacinda Ardern joined Mr Morrison in tribute.

"Our thoughts are with the people of Tonga," she said.

"Prime Minister Pohiva was an elder statesman of the Pacific ... he will be remembered for his lifelong commitment to championing democracy.

"He was also a powerful advocate for Pacific regionalism, demonstrated at the Pacific Islands Forum Leaders Meeting in Tuvalu where he advocated for climate change action and regional solidarity."

Pacific Islands Forum secretary general Dame Meg Taylor said Mr Pohiva showed great courage to attend the most recent meeting, four weeks ago, while very unwell.

"I think, in many ways, he came to say goodbye, to show his respect and solidarity with the Forum Leaders and to make a final resounding stand on issues close to his heart," Ms Taylor said.

"His attendance in Tuvalu was important for the outcomes we were able to reach, especially on climate change and West Papua.

"I will remember him as a fighter until the very end. May his legacy stand true and be an inspiration to the people across the Blue Pacific."

Mr Pohiva had struggled from a string of health problems in his elder years, and had already spent time in Auckland this year as he sought treatment for liver problems.

Australian Associated Press