• 4150 girls in Year 7 and 7550 in Year 8 start menstruating during the intermediate school year;
• About 1900 girls nationally or one in 16 girls start menstruating while of primary school age;
• Almost half of all girls (48.7 per cent) have started menstruation before they start secondary school.
"Previously we have not had formal data about what age New Zealand girls get their periods," Donovan said.
"[We] have had to extrapolate from small local studies and/or international data, which show variation between countries and do not reflect the NZ ethnic and socioeconomic context."
The data is from the 2014/5 NZ Health Survey, and included a specific question to female participants answering the sexual and reproductive health module about their age when they first started having periods.
"This new data will allow us to make comparisons with international data, which indicates that globally the age of first periods is decreasing," Donovan said.
"The reasons for this are unclear, however it is suggested that it may be due to the impact of environmental toxins, and/or increasing body mass index [BMI] in young girls."
The average age of a first period was 13 with almost 50 per cent of girls in primary or intermediate school at that age.
Donovan said the data meant New Zealand needed to target health education, resources and support to an even younger age group in order for Kiwi girls to be prepared to manage their periods without embarrassment and disruption to their schooling.
Her call for primary schools to ensure sanitary disposal units were provided in girls' toilets follows a situation in 2016 where an Auckland mother complained that her primary school-aged daughter [then a 9-year-old] had been asked by the school principal to stay home from school during her periods because it did not provide sanitary disposal units.
Previously the Ministry of Education has stated it does not have a role in ensuring that schools provide sanitary disposal units and had no plans to require this.
That story also highlighted the broader issue of Kiwi girls missing school through not being able to afford pads and tampons, Donovan said.
It comes as demand for sanitary products in primary schools increases, with many parents on low incomes struggling to afford the essential items for their children, KidsCan chief executive Julie Chapman said.
"A lack of access to sanitary items is a serious and hidden equity issue which needs to be addressed to support these young girls, particularly those of primary school age," Chapman said.
"It's really a matter of child rights that no girl, of any age, should miss school because her family could not afford menstrual products."
This year the charity has supplied 329 schools across the country with more than 16,000 boxes of pads, tampons and liners.
Papatoetoe West School teacher Lizzy Lockhart said it can be a frightening time for girls who start menstruating at primary school.
"There's still so much stigma around bringing it up and being the one kid in the class who has their period, and feeling like that means something is wrong with you," Lockhart said.
She said it's a huge relief for the pupils when she can give them free sanitary items from KidsCan.
"A lot of our kids don't come to school with lunch - and if their families can't afford lunch how will they go out and afford to buy sanitary products?
"I know when one girl wasn't getting stuff from our KidsCan supplies at school she was just using toilet paper."
Chapman said the charity added sanitary items to the list of school essentials it provided more than three years ago, after she found that girls were staying home from school because they didn't have any products.
"It is an absolute tragedy that girls are missing school because they have their period," Chapman said.
"Education is their ticket out of poverty, and they shouldn't be falling behind because of a lack of access to sanitary items."
