The people of Tonga have been plunged into mourning following the death of 88-year-old King Taufa'ahau Tupou IV.
King Taufa'ahau had been suffering from an unspecified illness
Public buildings are being draped in black and purple as a mark of respect.
King Tupou IV was absolute ruler of the South Pacific island nation for 41 years, making him the world's fourth longest-serving monarch.
His son, Crown Prince Tupouto'a, was sworn in as the new king at a brief ceremony on Monday, but it could be at least a year before a full coronation.
Correspondents say King Tupou IV was much loved by his people, but his death is likely to fuel calls for greater democracy.
King Tupou's death was reported late on Sunday at Auckland's Mercy Hospital, where he had been receiving treatment since April.
The Tongan government made a formal announcement shortly afterwards. "The sun has set in the kingdom of Tonga," it said in a statement.
The king's body will remain in Auckland until Wednesday, before being moved back to Tonga to lie in state.
Mourning has already begun and is expected to last up to a year.
King Tupou took over the monarchy in 1965, after the death of his mother, Queen Salote, and soon began modernising the archipelago's education system and infrastructure.
Throughout his reign, the royal family controlled Tonga's semi-feudal political system and most of the economy, which is dependent upon farming, fishing and remittances from expatriate Tongans.
The king made headlines around the world in the 1970s, when he became the world's heaviest monarch at over 200 kg (440 lb).
But in the 1990s he headed a national keep fit campaign and shed a third of his weight.
For most of his reign, King Tupou had the respect and loyalty of his subjects and other leaders in the South Pacific.
But in recent years, he has faced increasing dissent.
In 2005, thousands of people took to the streets to demand democracy and public ownership of key assets, in unprecedented public demonstrations.
South Pacific analysts say the king's death is likely to fuel demands for greater liberalisation in Tonga.
The king's death is the second blow to the Tongan royal family and the Tongan people in recent months.
The kingdom has only recently mourned the death of one of the king's nephews, who, along with his wife, was killed in a car crash in San Francisco in July.