NZ Herald's Adam Pearse reporting from Windsor Castle, London

Prime Minister Jacinda Ardern believes she may never witness an event as "extraordinary" as Queen Elizabeth's funeral that was held across London.

Monday's funeral and committal service, along with an earlier reception for world leaders, also provided her a valuable opportunity to speak briefly with United States President Joe Biden, Queen Consort Camilla and freshly minted UK Government ministers.

Upon leaving the committal service at Windsor Castle, Ardern was able to farewell King Charles III - urging him to take care of himself as the grief of losing his mother evolves.

Ardern spoke to the NZ Herald following a momentous day in London where it's likely millions - perhaps billions - around the globe lined the streets or watched from home as the late Queen was farewelled.

Under clear skies, Queen Elizabeth's coffin was transported by a state gun carriage - once used to move her father's coffin - from Westminster Hall where she had been lying in state for some days, to Westminster Abbey for a state funeral.

About 2000 guests filled the majestic abbey to hear funeral lead Dean of Westminster David Hoyle paying tribute to the Queen's "unswerving commitment to a high calling over so many years as Queen and Head of the Commonwealth".

"With admiration we recall her life-long sense of duty and dedication to her people…With affection we recall her love for her family and her commitments to the causes she held dear."

Ardern was seated very close to the Queen's coffin, seen here walking behind Britain's Prime Minister Liz Truss, (centre) and husband Hugh O'Leary (centre right). Photo / PA

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Ardern was seated in the nave next to Andrew Holness, Prime Minister of Jamaica and her partner Clarke Gayford, They were in the third row, a prime viewing spot, given New Zealand's position as a realm country. They had direct sight of the coffin, and the royal family.

Across the aisle, in the second row was Governor-General Dame Cindy Kiro, and her husband Richard Davies. Biden was in the 14th row.

Other New Zealanders at the funeral included former Governor-General Dame Sylvia Cartwright, Sir Don McKinnon - secretary-general of the Commonwealth of Nations from 2000 until 2008, and singer Dame Kiri Te Kanawa.

Victoria Cross recipient Willie Apiata was resplendent in military dress. Jacinda Amey, holder of the New Zealand Cross, followed in the procession.

About 2000 people packed out Westminster Abbey for the Queen's state funeral. Photo / AP

Following the hour-long funeral, the coffin was returned to the carriage before it was escorted almost 4km from the abbey to the Wellington Arch at Hyde Park corner.

The procession involved many military groups from Commonwealth countries, including several contingents from New Zealand's Defence Force.

Upon reaching Wellington Arch, the coffin was driven in a hearse to Windsor Castle - where a committal service was held in St George's Chapel.

An emotional and intimate affair, Ardern and Kiro were fortunate to earn an invitation to the 800-strong service that concluded with the Queen's coffin gradually descending into the royal vault to be buried with her late husband, the Duke of Edinburgh.

Ardern told the Herald the day was unlike anything she'd experienced.

"It was an extraordinary piece of history to be a part of and I doubt I'll see anything of that scale or nature in my lifetime.

"But at the same time, despite its scale, there were still these really intimate moments that would have felt familiar to anyone who's lost a loved one."

The Queen's coffin enters St George's Chapel for the committal service. Photo / AP

The 15th-century chapel is split in two sections; the nave where the majority of guests were seated, and the quire where the Queen's coffin was placed surrounded by senior members of the Royal family.

Again indicative of the priority placed on realm countries, Ardern sat just 2 metres away from the entrance to the quire and three rows back - directly opposite extended members of the royal family.

Kiro was even closer, sitting in the front row along the centre aisle and would have had among the best view from inside the nave.

After the service, Ardern was able to say a short goodbye to the new King.

"It was just simple, [I said] I hoped he would look after himself," Ardern said.

The encounter followed a 10-minute meeting Ardern had with King Charles III a couple of days ago when they had a general conversation about New Zealand and the Commonwealth, as well as speaking of the Kiwis attending the funeral.

Ardern also saw the King at a function he hosted the night before the funeral for the many world leaders who had travelled to pay their respects to his late mother.

There, she was able to speak to the King as well as Queen Consort Camilla, whom she had not yet spoken with since being in London.

US President Joe Biden accompanied by the First Lady Jill Biden at Monday's funeral. Photo / PA

She also shared a brief audience with Biden as they entered the function, along with a range of other leaders.

An interval between the funeral and the service also afforded Ardern the chance to speak with other members of the royal family, including the King's brother Prince Edward, the Earl of Wessex.

Ardern said her bus ride to Windsor Castle was one of the many features of the day that were the most touching for her.

"There were a huge number of people on the side of the road but I glanced up and I saw a woman standing in her window alone, dressed in a black frock, just with her head bowed, in complete solitary and clearly just standing there as a mark of respect.

"It was moments like that that I found probably the most profound in amongst everything else."

The Queen's death has prompted a discussion nationally as to whether New Zealand should remove the now King as head of state and become a republic - something Ardern has repeatedly ruled out under her Government.

Asked whether she believed the commemorations would make New Zealanders feel closer to the monarchy, Ardern would not speculate on behalf of all Kiwis.

Asked whether she personally felt closer to the monarchy after the commemorations, Ardern said she had been honoured to have relationships with the royal family as prime minister but didn't think that was necessary to feel an affinity to them.

"I have felt privileged over the years to get to know some of the individual family members just by being in the role and what it has shown me is a genuine affection for which New Zealand is held.

"But I think all New Zealanders see that, I don't know that you need to have that personal conversation or meeting to see that, it's very genuine."

King Charles III and Prime Minister Jacinda Ardern have had several meetings over recent days. Photo / Supplied

As she was speaking to the Herald, Ardern was driving to the airport before her flight to New York where she'll be attending this week's United Nations general assembly.

Despite the radical differences between the two overseas events, Ardern welcomed the challenge, invoking the spirit of the late Queen.

"I've got eight hours in front of me to switch that mindset and quite a bit of work on the flight so that'll shift my mind pretty quickly, but as the Queen herself once said to me, you just get on with it."