Papua New Guinea's first prime minister, Grand Chief Sir Michael Somare, has died at the age of 84.

Known as the father of the nation, Sir Michael led PNG into independence from Australia in 1975 and was one of the country's longest-serving politicians, with a career that spanned five decades.

The news was confirmed by members of his family.

He died early this morning after being diagnosed with a late-stage pancreatic cancer and admitted to hospital on February 19, his family members said in a statement.

"Sadly, pancreatic cancer is one of the most aggressive cancers that are rarely detected early. We as a family had only two weeks to look for possible treatments for our father," a statement from his daughter Betha Somare said.

Michael Thomas Somare was born on April 9, 1936 in Rabaul, a coastal town in what was then the Australian mandated territory of New Guinea, and raised in the East Sepik Province, a region he went on to represent in parliament.

In 1972, Sir Michael was elected as Chief Minister of the territory and pledged to lead it to self-government and then independence.

Just three years later, PNG gained its independence and Michael Somare became the country's first prime minister at the age of just 39.

"A lot of people in this country thought we wouldn't be able to do it, they were talking in terms of two or three decades, while I was talking in terms of two years," Sir Michael said at the time.

Despite his optimism, Sir Michael hadn't considered that he might be Papua New Guinea's first prime minister.

He had worked as a teacher and a radio journalist before entering politics in 1968.

Prime Minister Scott Morrison offered Sir Michael's family his condolences in a tweet, calling him a "great friend" to Australia.

"My heartfelt condolences to his family, Prime Minister Marape and the people of PNG. Australians stand with you in this time of sadness. May he rest in peace," the tweet said.

Move into politics came by surprise

"At one stage in Australia … I said I don't see it, I can't see myself out to be the prime minister. I think that was one of my quotation in Australian paper," he said.

"I never thought by forming the first political party, we would be able to see it through.

Sir Michael and his peers of that time were young, energetic, idealistic, and nationalistic, and had had enough of Australian rule.

"During that time I was not referred to by my name, instead I was given a series number in Australian colonial territory … we had few rights and our opinions were given to us," he reflected in 2017.

Papua New Guinea is a complex nation, with more than 800 different language and tribal groups.

PNG observers, like the ABC's former PNG correspondent Sean Dorney, say it was not an easy task to unite them and form one country.

"Sir Michael is absolutely unique and there is no-one else in the rest of the Pacific or Australia or New Zealand who has anywhere near the political career that he has had," Dorney said.

"It's not just he has been around for so long, but he actually dragged the country through to independence and I don't think people appreciate how difficult that situation was."

Sir Michael would go on to stay in parliament for 49 years.

He was prime minister on three separate occasions over that time, holding the top post for 17 years in total, over four terms.

He also served as the foreign affairs minister, leader of the opposition and the local governor of East Sepik Province.

Dorney said the senior PNG statesman made the most of his time in politics.

"Australia was pretty pleased to get out when it did, so I don't know if PNG could have kept the Australians here for much longer, because the enthusiasm in Australia to keep trying to run this incredibly complex and difficult country was not there anymore," he said.

"What Sir Michael proved is that it is possible to run this incredibility difficult and complex country."

His father Ludwig Somare was a police officer but he also held tribal leadership in the East Sepik bearing the name "Sana" which means peacemaker.

Young Michael was exposed to those traditional values early on in his life and they stayed with him during his political career.

Power struggle with Peter O'Neill

But Sir Michael's five decades in PNG politics were not without controversy.

He was ousted from parliament, and the prime minister's post in mid-2011, after being out of the country for several months for heart surgery in Singapore.

The Opposition had declared the prime minister's post was vacant because of his long absence, and Sir Michael was replaced by Peter O'Neill.

But four months later, the PNG Supreme Court ruled that the election of Mr O'Neill was unconstitutional, and called for Sir Michael to be reinstated.

However, Mr O'Neill still had the support of most MPs, leading to a months-long power struggle between the two men and constitutional crisis.

Both claimed to be the legitimate prime minister and each appointed their own governors-general, speakers of parliament and police commissioners.

The situation escalated when MPs supporting Mr O'Neill stormed government house , shouting "we're unarmed and we're the legitimate government".

Elections were held in mid-2012 and the people of Sir Michael's East Sepik province voted him back into parliament.

Five years later, his political career finally came to an end when he retired.

"We progressed through many waves and changes in the world, we survived our own bad decisions," he said in his last address to parliament in 2017.

"We have united at times when the world thought it was not possible to do so, we must be thankful and we must always count our blessings.

People in Papua New Guinea knew Sir Michael affectionately as "The Chief" and the "Father of the Nation" and he is one of only two people in PNG to be given the official title of "Grand Chief".

He was knighted by Queen Elizabeth II in 1990.

He is survived by his wife Lady Veronica Somare, who he married in 1965 and their five children.