A new report estimates the cost of mental distress among new parents carries a long-term social cost of more than $2 billion a year.

Clinical mental distress includes perinatal OCD, birth trauma and PTSD, postpartum psychosis, as well as the more commonly known postpartum depression, which affects about 15 percent of the population.

But the cost estimate rose to $4.3b when it considered sub-clinical distress, too - things like sadness, anxiety and trouble sleeping.

The report was commissioned by the Tindall Foundation, a private family philanthropic foundation, and written by Auckland-based consultancy Social Prism using data from sources including the Ministry of Health, Te Whatu Ora, and Stats NZ.

It found up to half of māmā and birthing parents - around 29,000 people annually - experienced mental distress during pregnancy and the early years.

Many of the costs of mental distress soon after childbirth were felt later through health, education, welfare, child protection and justice systems.

Ashlee McCormick, the foundation's donations programme manager, said the estimate was conservative, and likely an under-estimate.

"Probably the scale of distress is much bigger," she said. "Around half of birthing parents experience some level of distress, so it's about 29,000 women every year."

There was a gap in community care, she said, which could be improved with more funding and better national policies.

"When mums are well, they're better able to care for their babies, recognise their cues, it helps build a better bond between mum and baby," she explained.

"And that we know if that bond is strong and there's a trusted adult or caregiver figure, it helps with the social cognitive development of those babies, which helps you be more school ready, which leads to better educational outcomes."

It also reduced the chance children would grow into adults who interacted with the justice system, which could be "a hugely expensive exercise", she said.

Eli Maiava founded the NGO Whare Manaaki o Te Tai Poutini in Greymouth in 2020, and she's also tangata whenua co-lead of the support service Te Hokinga Mai, funded by the Tindall Foundation - both focused on supporting whānau wellbeing in the community.

She said community was often the first place families reached out to - and it would be more effective to invest in early support, than to pay the cost in the justice and health systems later on.

"Those first 2000 days are integral for brain development, and that leads us into education, into mental health, into decisions that we make as adults," she said.

"Working in these spaces and seeing the challenges that whānau face, it really frustrates me that all of the investment is at the other end, when all of the impact of not being supported in those early days comes to bear."

Maiava said support organisations needed funding and flexibility to cater to whānau in their communities.