A nationwide study on menstruation has found that about one in sixteen of all Kiwi girls get their period before leaving primary school.
Data from the Minister of Health's NZ Health Survey was examined by University of Otago's Dr Sarah Donovan, who is now calling on Pharmac to fund sanitary products for all school-age girls.
The survey data showed that about 1900 kiwi girls per year begin menstruating while still in primary school - some as young as nine - which is in line with a global trend of girls beginning menstruation at an earlier age.
Almost half of all girls have begun menstruation before starting at secondary school, and the average age is 13.
Dr Donovan said this New Zealand-specific data should show schools and the government that resources such as sanitary disposal units and pads belong in all schools.
"Previously we have not had formal data about what age NZ girls get their periods, and 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," she said.
"These new data will allow us to make comparisons with international data, which indicate that globally the age of first periods is decreasing."
Dr Donovan also said it demonstrates a need to begin sexual education at a younger age.
The reasons for girls beginning their period at a younger average age are unclear, Dr Donovan said, "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".
She is now preparing an application to Pharmac asking for funding for sanitary products for all school-aged girls.