The Pacific is in mourning after ‘Akilisi Pohiva, the prime minister of Tonga, a long-time pro-democracy campaigner and leader in the fight against climate change, died aged 78.

Pohiva had been ill for the much of the last year and was receiving treatment in New Zealand for liver disease. He was evacuated to Auckland on Wednesday suffering from pneumonia, Radio New Zealand reported. He died on Thursday morning.

Pohiva was a key figure in Tonga’s pro-democracy movement, which saw him regularly clash with Tonga’s monarchy over his more than three decades in politics. In 2014 he became the first commoner in Tonga to be elected to the position of prime minister by Parliament, rather than being appointed to the role by the king.

Over the course of his parliamentary career – which began in 1987 – he was dismissed by the king along with the rest of his party, imprisoned for contempt of parliament, and twice charged with sedition.

Partly because of Pohiva’s push for democratic reforms, Tonga became a constitutional monarchy in 2010 and now has representative elections for parliament, however, reverence for the monarchy remains strong and the country has strict sedition laws.

Tonga’s late prime minister Akilisi Pohiva (third from left) attended the Pacific Islands Forum Tuvalu in August. Photograph: Mick Tsikas/AAP

He was awarded the Defender of Democracy Award in 2013 by the Parliamentarians for Global Action, the first Pacific Islander to receive the prize.

On Thursday local media reported that Tongan parliament had been suspended indefinitely in light of the news.

Tributes were paid to the prime minister from across the region. Dame Meg Taylor, the secretary general of the Pacific Islands Forum called him a “fighter until the very end”, a “compelling advocate for democracy and freedom, and a kind man and principled man with great affection for Pacific people”.

“As one of the longest serving parliamentarians in our region, Prime Minister Pōhiva dedicated his life to the service and leadership of his country and people. 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,” said Taylor.

During his political career we saw him transition from firebrand activist to an elder statesman of the Blue Pacific Dame Meg Taylor

Ralph Regenvanu, Vanuatu’s foreign minister, who said: “My condolences for a good friend and principled leader” and Australia’s prime minister called him “a passionate advocate for his people, for his beloved Tonga and our Pacific family.”

Despite his ill health, Pohiva attended the Pacific Islands Forum in Tuvalu in August, telling the Guardian that he thought this year’s PIF would likely be his last but that he had been determined to attend because of the importance of the summit, especially due to climate change.

“My physical presence in the meetings is significant to me,” he said. “I’m sure it is mostly likely that this will be my last attendance at a forum so it was very important to me to be here.”

I mourn the passing of Tongan PM ʻAkilisi Pōhiva, who inspired the world with raw emotion at last month's Pacific Islands Forum in Tuvalu (which, despite his health, he attended in recognition of the urgency of climate action). We must honour his legacy by continuing this fight. pic.twitter.com/MRHysVAaDU — Frank Bainimarama (@FijiPM) September 12, 2019

Pohiva spoke passionately at the forum about the need for urgent action on the climate crisis, which is already having serious impacts on Pacific nations such as Tuvalu and Tonga, and called for action to end alleged human rights abuses in West Papua. Tuvaluan prime minister Enele Sopoaga said Pohiva had cried at the leaders’ retreat while talking about two young women who had presented to leaders on Monday about the impacts of the climate crisis.

Dame Meg Taylor said Pohiva “showed great courage and resolve to attend the meeting while he was 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.”