The ministry's National Screening Unit commissioned the study from Australian cancer researchers to see if the programme was achieving the main aim of breast screening programmes internationally - to reduce the breast cancer death rate.
The BreastScreen Aotearoa programme's clinical director, Dr Marli Gregory, said the study found that for women who have been screened in the programme, the rate of death from breast cancer is reduced by a third when compared to women never screened in the scheme.
One of the researchers, Professor David Roder of the University of South Australia, said: "This study shows that New Zealand's organised breast screening programme has been associated with clear and significant reductions in breast cancer deaths in New Zealand women."
But Otago University cancer epidemiologist Associate Professor Brian Cox said the size of the mortality reduction was "strange", based on the randomised, controlled trials of breast screening on which national programmes had been based.
"We have estimated that the maximum effect you could expect would be around 26 to 27 per cent in women aged 65 to 69. That would be the group who get the maximum effect."
Professor Roder said the field trials undertaken in the 1960s to 1980s used older technology and variable protocols; it was plausible that outcomes today could exceed those results. The NZ findings were similar to those for comparable programmes in Australia, the UK and Europe, which reported mortality reductions of greater than 35 per cent in screening participants.
Cancer charities say the New Zealand findings are important, particularly in light of the doubts cast by some on the value of screening.
"It's time for all those people who have sought to undermine our valuable breast screening programme to finally shut up," said Breast Cancer Foundation chief Evangelia Henderson. "[BreastScreen Aotearoa] does an amazing job and it's very frustrating when irrelevant overseas studies, particularly from countries like America, which simply doesn't have a national, co-ordinated screening programme, are used to claim that mammograms aren't effective.
"Mammograms save many lives, and it's great to at last have national mortality figures to back up what we know from the regional breast cancer patient registers.
"If you want to give yourself the best chance of finding breast cancer early and surviving it, sign up with BreastScreen Aotearoa and have regular mammograms."
Libby Burgess, chairwoman of the Breast Cancer Aotearoa Coalition, said the findings were powerful.
"They show how crucial breast screening is, not only in detecting breast cancer, but in ultimately saving lives."
Dr Gregory said although Maori women's rates of breast cancer incidence and death were higher than for other New Zealand women, the study confirmed that if Maori women's rates of screening could be increased to match the rest of the population, "we would see similar reductions in the rate of deaths from breast cancer".
Survivor owes her life to screening programme
Marilyn Whelan says she owes her life to the breast screening programme.
Now 67, Ms Whelan, a Remuera hair salon owner, was diagnosed with breast cancer in 2010. An abnormality detected in a mammogram done through the screening programme led to a biopsy.
Surgery followed, to remove a tumour the size of a pea in her right breast and affected lymph nodes, then chemotherapy and radiation therapy, plus ongoing tamoxifen.
Ms Whelan now considers the cancer to have been cured. She is in no doubt that screening saved her life and doubts she would have found the tumour herself.
"I have been having mammograms since I was 40.
"I think I did feel it a couple of months before [it was picked up on the mammogram]. I have that mindset that that wouldn't have been me, I wouldn't get breast cancer.
"Then I forgot about it till I went and had the mammogram. I just felt it in the shower one day. When I tried to find it again, it was so devious, I couldn't find it," she said.
"I just ignored it. I think I was relying on the mammogram, as most women do."
Ms Whelan urges all women to have regular mammograms.
The study
Compared those screened for breast cancer with those never screened - for all eligible women, and by ethnicity
The findings
• 34% reduction in rate of death from breast cancer among all women screened
• 28% reduction among Maori women who were screened
• 40% reduction among Pacific women who were screened.
