The Tongan Prime Minister and long-time democracy campaigner, 'Akilisi Pohiva, has died.

The 78-year-old had been medevaced to Auckland yesterday, suffering from pneumonia.

Starting life as a school teacher, Pohiva grew to become one of the stalwarts of Tonga's pro-democracy movement.

A constant thorn in the side of the monarchy and nobility, Pohiva's lifelong battle for representation has seen him fired from the public service and charged with sedition.

But he maintained support among the people, who repeatedly voted him into Parliament after he was first elected in 1987, making him the longest-serving people's representative.

In 1992, he told RNZ that democracy would eventually come to Tonga.

"Those people at the top outcast me. They have been trying to isolate me from the rest of my colleagues and from the people, but they could not. More and more senior people in the country have come to back up and support the movement and I am very confident change will happen."

And that change did happen.

Constitutional changes eventually cleared the path for him to become prime minister in 2014, before he was re-elected in 2017.

Pohiva has been in poor health in the past few years, and he spent much of this year in Auckland being treated for liver problems.