Through a private oncology specialist, the secondary school teacher staked her life on two incredibly expensive drugs.
The first was dabrafenib, a gene therapy treatment which essentially starves the cancer, but in most cases the melanoma eventually becomes resistant to.
At that point, immunotherapy drugs such as pembrolizumab, better known as Keytruda, become the brightest hope.
It works by "flipping a few switches in the immune system", in Renwick's words, so the body can recognise and attack cancerous cells.
Keytruda - which costs $8500 every three weeks - doesn't work for everyone, but it worked for Renwick.
Then she got angry again.
"I was getting better and I'd meet all these people [dying from melanoma] and they'd ask 'How are you getting the drugs? And I'd have to say 'I'm paying for it'," says Renwick, 48.
"And I knew they were thinking 'I can't [afford them]'."
So while still fighting for her own life, Renwick started fighting for the lives of others.
Keytruda is state-funded in Australia and the United Kingdom, but Pharmac (New Zealand's state drug funder) was not convinced.
It was all politics. We knew the only way was to put pressure on our politicians," says Renwick.
Pharmac cited a lack of evidence about the drug's efficacy and said evidence presented by pharmaceutical maker Merck Sharp and Dohme had not been peer-reviewed.
"It was all politics. We knew the only way was to put pressure on our politicians," says Renwick.
Though melanoma is treatable with surgery if caught early, Renwick says it is a "lonely disease" if diagnosed in the late stages.
"It moves quickly, very fast and nasty. You can become very isolated...there were a few support groups, or Givealittle pages to fundraise - but everyone was working in silos," says Renwick.
"There's a big reason for that...if you're fighting for your life, or the life of your son, your mother, you don't have time to be advocating for anything else. You're trying to keep them alive.
"I realised there was no strength in that. People had to stand together to make themselves heard."
The mother-of-three had the "crazy idea" of starting a petition to Parliament requesting extra funding for Pharmac so pembrolizumab can be made "available to all New Zealanders with advanced melanoma, regardless of socio-economic status".
She hoped for 5000 signatures. There were 11,085 and Renwick didn't believe the total, so she counted them again.
The petition was referred to the Health Select Committee, but in May the Government acted before submissions were heard - giving an extra $39 million in funding for Pharmac to fund Keytruda and Opdivo, another immunotherapy drug for advanced melanoma.
The U-turn was a tremendous victory for ordinary people.
Renwick is proud of what was achieved but quick to point out she couldn't have done it without her family (especially husband Wayne), close friends who nursed her and others suffering from melanoma, as well as medical experts who spoke out.
"I might have been the public face that people saw, but I wasn't standing there alone. There were lots of brave people who stood up with me."
For her, the decision to fight for Keytruda was more than just dollars and cents. Or dollars and sense. It was about New Zealand's place in the world.
"You hear the argument, 'we're only a small country, we can't afford everything'. That's true. We have a finite budget.
"But it all comes down to what we stand for as a country and what our vision is. That's where your budget comes from.
"It's no different to the budget of any organisation. It comes down to what your values are and then it's easy to make the decisions that have to be made.
"If we're going to make the choice, around all these diseases we call cancer, that we will let people die, while they can look across the Ditch to Australia and see people living, let's stand up and say it."
Challenging the power-brokers to make a different decision was simply participating in democracy, says Renwick.
"I sincerely believe in the democratic system. But the democratic system is not just electing someone every three years. That's just electing a dictatorship. We're part of the system where we hold our elected representatives accountable. That's all we did."
Sitting in the shade by the pool at her home in Tauranga, Renwick reflects on 2016 as the "strangest of years".
It didn't feel real when she was given a few weeks to live. It didn't feel real when she presented 11,085 signatures on the steps of Parliament. And it didn't feel real when the Government stepping in with more money.
That was the result of hard work by Renwick, and others, which she found exhausting. "Sharing a private pain in public is very difficult."
She also acknowledges some good luck as she keeps winning - touch wood - her own battle with cancer.
Winning the Herald's New Zealander of the Year award is "humbling" and she hopes it will bring closure on a stressful time.
Whatever happens in 2017, Renwick is determined to be grateful for each and every day.
"We drop into patterns where we work, work, work. But none of us know whether we'll be here tomorrow. It's enjoying life, even just noticing that it's a beautiful day."
The good news about Keytruda and her own health was tempered by the fact that friends she made during the campaign died waiting, or finally received the treatment, but it was too late to take effect.
And in some cases, Keytruda doesn't work.
But for every life cut short by melanoma, Renwick has heard stories where there is now hope.
"You used to get sent out the door and told there's nothing they can do for you. Now there is something they can do for you. And that's really neat."
The Herald New Zealander of the Year
Leisa Renwick
48, lives in Tauranga
Mother of three, married to Wayne
Maths teacher at Mt Maunganui College
May 2015: Diagnosed with Stage 4 melanoma and given weeks to live. Told no treatment available in public system. Started taking dabrafenib, a gene therapy drug, which costs $12,000 a month.
August: Tests showed her cancer had shrunk. But she was told the cancer would become resistant to dabrafenib.
October: She switched to pembrolizumab, better known as Keytruda, which costs $8500 every three weeks.
November: Interviewed in Herald series on funding for cancer treatment. Appalled by "have, have not" divide in health care.
"It seems to me in this country the rich people are able to get treatment and the poor people aren't. They are sent home with lots of painkilling drugs. They are sent home to die. I think that's really wrong."
Starts petition to Parliament requesting more funding for Pharmac to provide pembrolizumab for those with advanced melanoma.
March 2016: Presents petition of 11,085 signatures on steps of Parliament.
May: Government announces $39 million funding boost for Pharmac.
June: Pharmac announces Keytruda will be publicly funded from September.
August: Renwick makes submission on petition to Health Select Committee, unusual because the petition was already successful.
December: Named the Herald's New Zealander of the Year.
The other 2016 New Zealand Herald New Zealander of the Year finalists were: Helen Kelly; Professor Patria Hume; Taika Waititi; Peter Boshier; Jason and Major Timms; Eliza McCartney; Peter Beck, an anonymous mum who brought school seclusion rooms into the light; and Te Whanau o Te Puea Marae.