As Tonga mourns the loss of King George Tupou V, New Zealand paid tribute to the 63-year-old monarch who encouraged political reform.
The king passed away with his brother, the crown prince Tupouto'a Lavaka Ata, reported to be at his bedside after being rushed to hospital in Hong Kong.
Prime Minister John Key said he was saddened by the king's death and paid tribute to the reforms he introduced in the impoverished island nation of 115,000.
He acknowledged King Tupou's contribution in steering Tonga toward democracy.
Three days before his coronation, the monarch announced he would relinquish most of his powers 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.
"New Zealand has a strong, close and friendly relationship with Tonga, with deep ties at a personal level and frequent contact at all levels of government."
An official announcement on Tongan radio by Prime Minister Lord Tu'ivakano said the monarch's death cast an "unexpected black storm cloud" over the kingdom.
The cause of death was not immediately known but he underwent surgery to have a kidney removed in Los Angeles last year after a cancerous tumour was discovered.
King Tupou was sworn in as king of the last monarchy in the South Pacific in September 2006, with the need for reform taking on added urgency when riots two months later left eight people dead and much of downtown Nuku'alofa destroyed.
Before ascending to the throne, the monarch was best known for his globe-trotting lifestyle and love of elaborate uniforms, including colonial-era pith helmets and jackets with elaborate gold braiding.
He often wore a monocle and was driven around the capital Nuku'alofa in a black London taxi, sometimes attracting criticism for being out of touch with his people.
But he was also a keen rugby fan and delayed the crowning ceremony to work on reforms, while toning down his playboy prince lifestyle.
When he was finally crowned in an elaborate Tongan ceremony on 2008, more than 200 nobles and chiefs presented dozens of slaughtered pigs and hundreds of baskets of food in tribute.
The king wasn't married, but had an illegitimate daughter not eligible for royal status.
He is expected to be succeeded by his younger brother, the crown prince, who is also the Tongan envoy in Canberra.