Cancer disgrace: PM Jacinda Ardern's hand-written letter to 12-year-old Lilly Vining

Prime Minister Jacinda Ardern has shared her own cancer experience in a hand-written letter to 12-year-old Lilly Vining whose dad Blair is dying of cancer after being let down by the public health system.

Ardern's letter talks of the loss of her nana and includes details of the Government's plan to tackle New Zealand's alarming cancer death rates - ahead of Health Minister David Clark's public announcement scheduled for next week.

Lilly - a rugby fanatic like her dad - wrote an open letter to Ardern and read it to the public in a video that went viral online earlier this month.

"Before you were Prime Minister you promised New Zealanders world-class cancer care including a national cancer agency.

"If you do what you promised it would tick the last thing off my dad's bucket list and help many more New Zealanders," Lilly read, as footage capturing precious moments spent with her dad, who she will soon lose, showed in the video.

Today, with the agreement of Lilly's family, the Herald can reveal Ardern's response.

The Prime Minister begins by sharing her own experience with cancer.

"I lost my nana when I was your age, and I felt like the world was closing in on me, so it's hard for me to imagine how you are feeling right now," Ardern wrote.

Precious moments of Lilly with her dad Blair. Photo / Supplied

She says how important it is New Zealand has a specific plan for cancer but is unsure if an agency independent from political interference would be better equipped to address the issues than the Ministry of Health.

"We made that promise when we were opposition (so before we were in Government) and we were worried the [former] Ministry of Health had lost lots of the resources it needed to do a good job and make sure we fix all of the issues stopping people from getting the cancer care they need," Ardern wrote.

She said her team were doing lots of work but it was taking time.

"When the minister shares the cancer action plan with us, that is when we'll be able to say who will be able to do the best job possible of making sure it's delivered properly," Ardern said.

Blair and Melissa Vining with their two daughters Lilly, 12, and Della-May, 17, have had their world tipped upside down following Blair's deadly cancer diagnosis. Photo / Supplied

Lilly's mum Melissa Vining told the Herald her daughter was thrilled the Prime Minister took the time to write a hand-written letter but their family was "devastated" by Ardern's uncertainty around the agency.

"She is going back on her promise and that's heart-breaking.

"The agency needs to have independent funding, be free from political interference, and set clear targets that Government and DHBs are accountable for," Melissa said.

Clark is due to make a public announcement on the Government's cancer plan - which has been worked on for six months - at some point next week.

A spokesman for Clark said he would be reviewing the plan this weekend ready for an announcement next week.

Tonight, hundreds of New Zealanders are continuing to fight for a cancer agency as they gather in Invercargill to join Lilly's dad Blair in one last effort to see change before he dies.

A petition signed by more than 101,388 New Zealanders, as of noon on Friday, will be handed to National Party health spokesman Michael Woodhouse at 7pm tonight.

Woodhouse will also present a speech expressing his support for the agency.

Political leaders, All Blacks, cancer specialists and victims of the deadly disease will all be there to support the Vinnings.

"We just want Blair to know change is coming before he dies," Melissa said, while holding back tears.