As the numbers of New Zealand women aged under 45 being diagnosed with breast cancer grows, the Breast Cancer Foundation has renewed its call to drop the age of eligibility for free breast screening.

Mammograms are free every two years for women aged 45 to 69 years through the national screening programme, BreastScreen Aotearoa.

Ministry of Health data shows an increase in the number of breast cancer registrations in young Kiwi women since 1995 when the screening service was introduced.

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In 1999, 499 women aged 25 to 45 were registered as having breast cancer. In 2012, rates increased 59 per cent to 797.

The news 36-year-old Cabinet Minister Nikki Kaye had been diagnosed with breast cancer sent shock waves around the country this week. The Auckland Central MP is expected to be away from Parliament for months.

New Zealand Breast Cancer Foundation would like to see free screening start at 40, chief executive Evangelia Henderson said.

"We have a very good screening system but the foundation is pushing to extend the screening age from 45-69, to 40-74."

The foundation advised women aged 40 to consider having mammograms, despite the current cost.

"The GP, if they're on to it, should be talking to women about a bunch of stuff, one of them being breast health. It's a milestone," Henderson said. "$150 or so is not a lot of money when it's about your health."

Ministry of Health acting chief medical officer Andrew Simpson said the increase in the number of young women with breast cancer would be affected by population growth.

The rate of women aged 40 to 44 with breast cancer had increased since 1999, but the rate of women aged 25 to 39 had remained static or decreased.

The screening programme would have impacted the number of breast cancers diagnosed, but there wasn't clear evidence it should be extended, Simpson said.

"Internationally it is not recommended that women under the age of 45, at average risk of breast cancer, are screened for breast cancer," he said.

"This is because the rate of breast cancer is much lower in women in their early 40s, meaning that the benefits of breast screening are outweighed by the harms."

The ministry would, however, continue to monitor international reports on the effectiveness of screening for women below 45.

Breast cancer initiatives

• The $59 million a year national breast screening programme was rolled out in 1999. It has reduced the breast cancer mortality rate for women screened for breast cancer by a third.

• The National Screening Unit funds $3.45 million of screening support services a year which help a wide variety of women to attend appointments.