The king of New Zealand's Maori died on Friday after heart surgery, with aides saying the beloved 69-year-old monarch, praised as a symbol of national unity, had "passed to the great beyond".

Spokesman Rahui Papa said King Tuheitia died peacefully surrounded by family, just days after celebrating the 18th anniversary of his coronation.

A symbol of Maori identity and kinship, Tuheitia occupied a largely ceremonial role and for most of his reign only spoke publicly once a year.

Nonetheless, he helped put New Zealand's rich Indigenous heritage at the centre of public life.

He campaigned to reduce high rates of Indigenous incarceration and for whales to be granted legal personhood, hoping to protect them from the ravages of evermore polluted and warming seas.

"The death of Kiingi Tuheitia is a moment of great sadness," Papa said in a statement. "A chief who has passed to the great beyond. Rest in love."

After a string of health problems, Tuheitia had been in hospital trying to recover from heart surgery when he died.

His body was brought on Friday afternoon to lie in state at a marae -- ceremonial grounds -- near the north island town of Ngaruawahia.

Black-clad mourners flocked to the site as news of the king's death spread.

Kawakawa leaves

New Zealand's Maori currently make up about 17 percent of the population or about 900,000 people.

Women carried wreaths of kawakawa leaves on their heads and were cloaked in patterned shawls. Some prayed gently as they placed bouquets of flowers.

Many were marked with facial tattoos -- an indelible statement of pride in their Maori heritage.

They passed under an ochre-red archway, ornately carved with beaked figures, and into the ceremonial grounds. 

Inside, leaders sang songs of bereavement -- the dirges and polyphonic melodies that have marked Maori funerals for generations.

A royal spokesperson told AFP that Tuheitia will lie in state before being buried on Wednesday at Mount Taupiri, the sacred "embracing mountain" that is the final resting place for Maori royals.

The process of electing his successor will be guided by a privy council.

During Tuheitia's past periods of ill health, his eldest son Whatumoana Te Aa Paki took over official duties.

'Immense fondness'

King Charles III, New Zealand's formal head of state, led the tributes, saying he was "shocked" having recently spoken to Tuheitia by phone.

"My wife and I were profoundly saddened to learn of the death of Kiingi Tuheitia," he said in a statement.

"I had the greatest pleasure of knowing Kiingi Tuheitia for decades."

The UK monarch said he remembered with "immense fondness" his meetings with Tuheitia in New Zealand in 2015 and at Buckingham Palace last year.

New Zealand Prime Minister Christopher Luxon praised the king's "unwavering commitment to his people" as he ordered flags on government and public buildings to be flown at half-staff.

"Today, we mourn," Luxon said in a statement from Tonga, where he is attending the Pacific Islands Forum. 

The Kiingitanga -- Maori King movement -- was founded in 1858 with the aim of uniting New Zealand's Indigenous Maori under a single sovereign, in the face of British colonisation.

The position has significant political and symbolic weight, but carries no legal status.

Tuheitia was the seventh Kiingitanga monarch.

Whales' rights

In 2006, Tuheitia succeeded his mother, Queen Te Arikinui Dame Te Atairangikaahu, who held the position for four decades.

During his reign he held countless ceremonies, visited the United Nations, met Pope Francis at the Vatican and represented a New Zealand delegation at the Olympic games in Paris.

Tuheitia called a 'hui' (national meeting) in January, when he asked for unity among Maori in response to concerns that the policies of the coalition government are undermining Indigenous rights.

In March, Tuheitia made international headlines with an impassioned call for whales to be granted the same legal rights as people, in a bid to protect vulnerable species.

He wanted the marine mammals to have inherent rights, such as having a healthy environment, to allow the restoration of their populations.

Chief of staff Ngira Simmonds told local media he would miss the king's love of music and community.

"I think what he will leave behind for this nation at large was his strong belief for unity, that unity is the best way forward for us as a nation," Simmonds said.

Around half a million New Zealanders tuned in to watch the last funeral of a Maori monarch, when Tuheitia's mother was buried in 2006.

Maori expert Carwyn Jones told AFP, saying thousands would likely attend in person.

"Here is an opportunity to see (the Kiingitanga) operating and being a focal point for Maori."