Tampons, pads and liners are necessities for women, but many struggle to afford them.
On average, the price reduction on pads and tampons could save women between $6 to $18 a year, depending on which products are used.
A Countdown spokesperson, Kiri Hannifin, said she took an interest in period poverty while working for Women's Refuge.
She said abusive partners would often use sanitary items as a tool of violence.
"Male abusers were withdrawing sanitary items from their partners and from their daughters as part of the violence so that, not only were there access issues in terms of money, but actually it was a weapon against the women as well - whereby they wouldn't be able to go to work and the girls wouldn't be able to go to school as part of his controlling behaviour."
The Labour MP for Manurewa, Louisa Wall, said that despite the price cut, there may be pockets of the community who still could not afford them.
She said that was a huge worry, because period poverty was forcing women to use unhygienic methods to manage their bleeding.
"Socks stuffed with newspaper, women trying to recycle tampons and wearing pads for multiple days. We have a problem, and we are starting to quantify how large that problem is, so an ability for all those interested in this issue to come together has been incredibly valuable."
Ms Wall said period poverty was also having an impact on teenagers, who missed school because they could not afford to buy tampons, pads and liners.
When added up, she said, the time taken equated to one missed school term per year.
The principal of Tarawera High School in Kawerau, Helen Tuhoro, said that was a major worry.
She said period poverty was a significant problem for the girls in her community.
"Our students in particular, it is a difficult thing for them to deal with out of school and often find it difficult or embarrassing to ask for those kind of things."
Major Pamela Waugh from the Salvation Army said she was grateful the problem of period poverty was finally being addressed.
"We know that girls are getting into trouble in the communities so we've heard stories of shoplifting, which means they get arrested, they get a criminal record and that can tag them for quite a while," Ms Waugh said.
"We also know that girls are at the risk of infection through using unsanitary products and that's going to effect their future fertility or their future health and well-being."
Countdown's price reductions came into force today, and while politicians, principals and activists applauded the move - they said there was always more to be done.