The government has gazetted March 1 and March 12 as public holidays to mourn Sir Michael.
"We are now saddened to celebrate his life, it's been a long journey. The whole nation will be in big mourning for the next week or so," said Sir Arnold Amet, who was attorney general in Somare's last government.
He said the passing of Sir Michael was a blow for the country, so soon after the death of another revered former prime minister, Sir Mekere Morauta.
"Sir Michael was like an anchor, an anchor for the nation," said Dame Carol Kidu, a long-time colleague and former MP, who is one of just a handful of women to have been elected to PNG's parliament.
Dame Carol said the Grand Chief was also instrumental in her becoming a politician, and stood by her despite opposition from many male colleagues to an independent woman MP becoming a cabinet minister, Radio New Zealand International reported.
"They said she is just one person. She brings no numbers to your cabinet, it's not fair. Always on both occasions, both elections when we went in he would say to them 'Gentlemen it is not negotiable - she is in my cabinet.' And I think a lot of people don't realise he was a champion for women."
Sir Michael was a Christian of deep faith and his daughter, Betha Somare, issued a statement saying that he was given his last rites and anointing by Cardinal John Ribat, the head of PNG's Catholic Church.
"Sir Michael was a loyal husband to our mother and great father first to her children, then grandchildren and great granddaughter," she said.
"But we are endeared that many Papua New Guineans equally embraced Sir Michael as father and grandfather."
Somare, PNG's first post-colonial prime minister, died in Port Moresby on February 26 aged 84 after a short battle with pancreatic cancer.
Somare was PNG's longest serving prime minister and was known as the Grand Chief and father of the nation.
He had other stints as leader, serving a total of 17 years in that role before retiring from politics in 2017.
"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.
"Sir Michael was a loyal husband to our mother and great father first to her children, then grandchildren and great granddaughter.
"But we are endeared that many Papua New Guineans equally embraced Sir Michael as father and grandfather," the family statement said.
Australian Prime Minister Scott Morrison tweeted his condolences on Friday morning saying Somare had been the founding father of democratic and independent PNG and had been "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."
New Zealand Prime Minister Jacinda Ardern said Sir Michael was not only seen as father of the nation in PNG but he also had a leadership role as a statesman of the Pacific.
"He will be deeply missed," she said in a statement.
Former Australian prime minister Kevin Rudd said Sir Michael had been a friend of his for the last 25 years and more importantly had been a friend of Australia throughout his political career.
Somare and his wife Veronica had been preparing to go overseas for treatment, his family said earlier last week but after assessing options had placed him in the best palliative care available in PNG.