Downing is speaking out to encourage people to seek a second opinion if doctors fob off genuine concerns.
"People have to go with their own [instinct] rather than thinking their doctor is a god."
His comments follow a Herald series which documented numerous cases where cancer had been missed by health professionals or treatment delays had led to patient deaths.
New Zealand has one of the highest rates of bowel cancer in the developed world. About 3000 new cases are diagnosed annually and the disease kills around 1200 people each year.
A national bowel screening programme for people aged 60 to 74 is operating at eight district health boards and will be extended nationwide by mid-2021.
Although Downing lives in an area covered by one of the DHBs already part of the programme, it started four months after he was diagnosed.
He first sought a GP's advice over bleeding from the bottom in December 2016. The GP thought it was haemorrhoids.
"He couldn't find any active bleeding on the outside. He prescribed nothing and said come back if it happens again.
"It did continue and I went back. He prescribed cream and other bits and pieces. Over 15 months I went to the doctor for one thing and another 13 times.
"Each time, even if it was a flu injection or carpal tunnel injection, I said I had this bowel bleeding. I was never given a stool test or a blood test and he never said anything about a colonoscopy."
In August 2017 Downing demanded to see a specialist and was referred to the hospital. Four months later, he saw a hospital doctor who removed some haemorrhoids.
The bleeding worsened over the following fortnight but Downing's GP said to wait another six to eight weeks for it to settle.
"I went straight home and rang the hospital."
An appointment was arranged for nearly three months later, March 2018. A colonoscopy was booked for early April.
The colonoscopy couldn't be done because the bowel was blocked by a tumour. Instead, a CT scan was done, which showed cancer cells had spread.
A hospital doctor phoned Downing with the results: "He said, 'It's as bad as it gets. It's started in your bowel and gone to your liver and your lungs'. I said, 'How long',and he said, 'I can give you a couple of months'. I was shellshocked."
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Downing was put on chemotherapy in May last year and said he had received exemplary care from the hospital's oncology department. The drugs had shrunk his bowel tumour but left him with numb fingers, toes and feet - nerve damage is a known side-effect for some patients.
He has survived longer than the predicted two months, but the chemo will end soon, after which his only option is to pay privately for the drug Cetuximab, which is not state-funded for bowel cancer.
It could extend his life, but the retired farmer said he was reluctant to pay the likely cost of about $40,000 for six months' treatment.
Downing said he had complained to the general practice, which he asked not be named, but had not contacted the Health and Disability Commissioner, preferring to concentrate on his health battle.
"One of the [medical clinic] partners rang and apologised. My doctor said had he found it earlier it would have been curable. My oncologist said the same thing."
In an email to Downing's daughter, a member of the medical clinic wrote: "I am sorry that your father did not get referred for colonoscopy earlier and I'm sorry that he and your family have had to suffer for this. In our GP education group I have made the other doctors in [the area aware of the] importance of early referral in cases like these and hope that we will improve our care of similar patients in future."
The clinic declined to discuss the case with the Herald.
"The family's concerns were investigated, and responded to, a year ago and we have always been willing to discuss them further with the patient and his family."
The Nelson Marlborough DHB said it did not comment on patients' care, but urged Downing to make a formal complaint.
"DHBs ask patients to contact them directly with complaints so that someone can have a conversation with the patient, hear them out, then log a complaint and look into it properly."
Health Minister David Clark said the health system needed to lift its game in cancer care.
Most cancer patients received timely, good care, but waiting times for some were too long. The ministry was developing recovery plans with those DHBs falling behind on waiting times. He had instructed the ministry to prioritise its cancer work.
