The 52-year-old prince is currently based in Canberra, acting as Tonga's High Commissioner to Australia for a number of years.
The youngest of the late King Taufa'ahau Tupou IV's children, he is no stranger to politics, having been the island nation's appointed Prime Minister in 2000.
He also held the portfolios of defence minister and foreign minister. In 2006 he stepped down as Prime Minister.
In his early twenties he joined the Tonga Defence Services.
The crown prince is married with three children. His eldest son, known by the title of Ulukalala, now steps up as the heir to the throne.
Late King 'a remarkable man'
Governor-General Lieutenant General Sir Jerry Mateparae also spoke about the King's work in progressing democracy in Tonga.
"While mindful of the importance of preserving Tonga's heritage and traditional customs, the King played a central role in promoting democratic change, including accepting the Tongan Parliament's choice of prime minister and being guided by his ministers in matters of governance"
Sir Jerry also reflected on Tonga's strong relationship with New Zealand, which he said was underpinned by a shared Polynesian heritage and a wide array of people-to-people links.
"The more than 50,000 New Zealanders of Tongan descent have given much to our country's society, economy, culture and sport and will mourn the King's passing," he said.
"At this sad time, on behalf of all New Zealanders I extend our sincerest sympathies to the King's family and the people of Tonga."
Meanwhile, Auckland Mayor Len Brown has said his office will be working with the leaders of Auckland's 40,000-strong Tongan community to decide how the city should commemorate the King's life.
"We all remember of the sea of Tongan flags during the Rugby World Cup, and the strong presence of our Tongan community throughout the region, particularly at events like Pasifika and Polyfest," Mr Brown said.
"I know the King's death will be deeply felt by the Tongan community here. My thoughts and the wishes of the wider Auckland community are with them and Tongans everywhere at this time."
Wayne Brewer, who first met King George Tupou V at King's Prep boarding school in 1958, described him as "remarkable man who has done amazing things for his people and his country".
"There are not many monarchs in the world who can bring about full democracy to their people - that takes a lot of doing and he did it very quickly."
Mr Brewer said his friend felt it was the right time for democracy in Tonga and the "right thing to do".
"He often said to me that he felt that it was his duty to look after the people, rather than the other way around."
He said the king was a proponent of education and tourism in his home country and did a lot of charity, Mr Brewer said.
"We all found him as a very kind, caring and generous man, and feel hugely honoured to have known him."
Mr Brewer said the King has kept in contact with his classmates and often visited them in Auckland.
He last saw the King at a 50th reunion at King's College at Cotter House, Remuera in November last year.
"I had an email from him only a week ago ... he didn't mention anything about being in hospital, he wouldn't speak of himself being ill."
Mr Brewer was disappointed with criticism over the King's alleged "extravagant lifestyle", such staying in upmarket hotels as opposed to the Tongan residence in Auckland.
"He has chosen to stay in hotels over the last 40 years because it was more economical than the huge costs running the Tongan house 'Atalanga' in Epsom."
The King made the headlines in New Zealand last year while trying to sell the $9 million official royal residence in Auckland, however the sale was blocked by protests.
Mr Brewer also defended the King's use of London taxis, saying he had a stiff leg as a result of a motor scooter accident in his 20s, and found the vehicles easy to step in and out of.
Mr Brewer said the King was a lover of animals, especially dogs, owning Jack Russells, Labradors and a schnauzer.
He spoke eight languages and was an accomplished cellist and pianist, Mr Brewer said.
Tongan media reported that he met Pope Benedict XVI on February 24 in the Vatican in Rome - and gave the Pope a signed picture of himself.
Democracy his "enduring legacy"
New Zealand Prime Minister John Key said King Tupou V will be "sadly missed by the Tongan community in New Zealand" and his thoughts are with the people of Tonga.
"I would like to acknowledge the very valuable contribution the King has made in steering Tonga towards democracy and hope this work will continue."
Three days before his coronation, the King announced that he would relinquish most of his power and be guided by his Prime Minister's recommendations on most matters.
"He believed that the Monarchy was an instrument of change and can truly be seen as the architect of evolving democracy in Tonga.
"This will be his enduring legacy," Mr Key said.
Labour leader David Shearer also expressed his sympathy and condolences.
"Our thoughts are with the thousands of Tongans living in New Zealand and the Tongan people themselves who loved their king," Mr Shearer said.
"King Tupou was an active supporter of change. He was known for his colourful style and charming personality.
"I am sure many will share in Tonga's grief on this sad day."
Tonga mourns
The news of the King's death spread quickly on Twitter early today, and many Tongans paid tribute.
Christinah Lataisia said: "Black and Purple begin to flood the fales in Tonga to mourn the passing away of our King."
Tessi Leila Tolutau said: "I am saddened by the news, our beloved King of Tonga has passed away ... our country has gone thru major losses in the past couple years."
Another resident said: "Sad day for our little Kingdom of Tonga. RIP King George Tupou V."
King Tupou took reign after the death of his father, King Taufa'ahau Tupou IV, in 2006.
In 2008 he marked his coronation with a $3 million, five-day event which was attended by thousands of people, including the Sultan of Brunei and then New Zealand Prime Minister Helen Clark.
Ms Clark also paid tribute to the late monarch on Twitter.
"Sad news for Tonga with the death of King George Tupou V. I attended his coronation in Nuku'alofa in 2008. RIP."
In September last year, the King received the Grand Cross of the Order of Merit of the Hungarian Republic from Hungarian president Pal Schmitt.
In February he received the Knight Grand Cross of the Royal Order of Francis I from Prince Carlo, Duke of Castro.
