Queen Elizabeth II, the UK's longest-serving monarch, has died at Balmoral aged 96, after reigning for 70 years.

Her family gathered at her Scottish estate after concerns grew about her health earlier on Thursday.

The Queen came to the throne in 1952 and witnessed enormous social change.

With her death, her eldest son Charles, the former Prince of Wales, will lead the country in mourning as the new King and head of state for 14 Commonwealth realms.

Prime Minister Jacinda Ardern has expressed New Zealanders' deep sadness at the passing of Queen Elizabeth II, describing Her Majesty as a monarch with an unwavering sense of duty.

'I know that I speak for people across New Zealand in offering our deepest sympathy to members of the Royal Family at the passing of the Queen. To us she was a much admired and respected monarch, to them she was a mother and grandmother,” says Ardern.

'The Queen was a much loved and admired monarch, whose record reign of 70 years is an absolute testament to her, and her commitment to us all. She was extraordinary.

'People throughout the world will be feeling an acute sense of loss at this time and New Zealanders most certainly share that grief. The Queen was a much respected constant through unprecedented global change.

'The Queen visited New Zealand on ten occasions, with that notable first tour over the summer of 1953-54 when she and Duke of Edinburgh visited 46 centres and attended 110 functions.

'She was here to celebrate with us at events such as the 1974 Christchurch Commonwealth Games and the 1990 Auckland Commonwealth Games. She also mourned with us when we were hit by terrible tragedies such as the Tangiwai rail disaster and the February 2011 earthquake.

'I know a number of New Zealanders who had the privilege of meeting Her Majesty were struck by her keen interest, warmth and sense of humour. I remember in my very first meeting with Her Majesty being humbled by her intimate knowledge of New Zealand and its triumphs and challenges.

'I presented her with a gift from a New Zealander who had kept a photo of her visit more than 50 years prior. She recalled where it was taken and even what had made her laugh at the moment the photo was taken.

'We will make arrangements for a State Memorial Service and many communities around the country will also want to show their respects and pay tribute at their own local events.”

The Prime Minister's Office says all decisions will be made by the prime minister but the current planning includes:

• A period of national mourning starts immediately following the announcement of the Queen's death and will continue until after the New Zealand State Memorial Service. The New Zealand service would happen after the Queen's funeral in the UK - likely to be about day 10 following her death.
• The New Zealand flag will be flown at half-mast from the announcement of the death up to and including the day of the funeral, except on Proclamation Day - the day the new Sovereign is announced officially when flags are to be flown from the top of the mast.
• Condolence books will likely be opened at Parliament and the National Library, as they were following the death of HRH The Duke of Edinburgh, and by local councils around the country.
• There will be gun salutes
• Places will be designated for members of the public to leave flowers

New Zealanders will receive full information and updates at the time via the Governor-General's website, it says.

Flags will fly at half mast to mark Her Majesty's death. Further information about mourning observances will be available on the Governor-General's website, at www.gg.govt.nz

National Party mourns death of Queen Elizabeth II

"On behalf of the National Party, I express our sorrow and sadness at the news of the death in Scotland today of Queen Elizabeth II, ending a remarkable reign and a lifetime of service," says party leader Chris Luxon.

"On the recent occasion of The Queen's 70th Jubilee, I said that her unflinching dignity, compassion, and selflessness had given the Commonwealth a sense of security throughout her reign.

"The strength and stability of Her Majesty's leadership of the Commonwealth was a reassuring anchor for New Zealand and New Zealanders in uncertain and changing times. Through both the tumultuous and the good, her dedicated service embodied the values of duty, commitment, and strength.

"I send my condolences to her family, who will also be mourning the loss of a beloved mother, grandmother, and great-grandmother, including to our new head of state, King Charles III."

ACT reacts to news of Queen Elizabeth's passing

'On behalf of the ACT Party, I would like to express my sincere condolences to the Royal Family for the loss of a true leader and matriarch,” says ACT Leader David Seymour.

'Her Majesty Queen Elizabeth II will be remembered for her lifelong dedication to public service. She has selflessly served the commonwealth every day of her life.

'She created a long period of stability for the Monarchy and the commonwealth, even as the world changed dramatically around her.

'It is no small feat to be the longest-lived and longest-reigning British monarch and the longest serving head of state in history.

'It is her work supporting charities that will be her legacy. She has supported hundreds of charities and is estimated to have helped raise well over a billion dollars to worthy causes. She has done more for charity than any other monarch in history, potentially any other individual.

'Her Majesty will be remembered fondly by New Zealanders and all in the commonwealth. We thank her for her selfless and tireless service.”

Taxpayers' Union pays tribute to the Queen

The New Zealand Taxpayers' Union joins those around the world in mourning the death of Queen Elizabeth II, the Queen of New Zealand.

'Queen Elizabeth was the greatest public servant in the Commonwealth," says Jordan Williams, a spokesperson for the Union.

"We join the whole nation in thanking her for a life of exemplary service.”

As with other taxpayer groups in the Commonwealth, the Taxpayers' Union will be suspending its campaign activities during a period of mourning.