A lack of funded services resulted in delays identifying and treating that depression, and for many of 93 per cent of affected women there few affordable treatment options available, she said.
"Not only does this mean an increase in severity of her illness and risk of suicide, but it also puts her child at more risk of mental health issues, learning difficulties, addictions and delinquency."
Health boards had different maternity mental health services, ways of funding them and qualifying criteria, but the funding came from other budgets such as mental health or maternity, Ms Paterson said.
"The issue is that there has never been Government funding, ever, and it leaves women vulnerable to going untreated and turning up later requiring treatment for mental health issues."
Ms Paterson is a former mental health nurse and the founder of Mothers Helpers, a non-funded support group.
Whangarei woman Shannon Schwander said she started experiencing depression when she was eight months pregnant.
"I couldn't bond with my baby after birth. I felt nothing when I looked at my baby. All I wanted to do was sleep.
"I felt he was an inconvenience. All he did was cry when I was tired and wanted to sleep. I couldn't stop crying, I didn't want to talk to anyone."
Ms Schwander felt like that for six months before someone realised how desperately she needed help - when she told her aunt she wanted to throw her baby out of the window.
It led to her being taken into Mental Heath Services care, being sectioned and hospitalised.
"I remember how it felt when those big doors shut behind me," she said.
Ms Schwander, who now has two children, described her 7-year-old as the light of her life.
She feels let down by services - including midwife and GP - that did not pick up her condition, and believes she lacked support that might have prevented her being involuntarily "locked up" for post-natal depression.
"Midwives aren't educated about it. Hospitals don't support mums enough.
"They just release mums with babies and say good luck, see you later - and expect us to be okay.
"I hated that I was made to feel like being a mum was 'normal' and if I couldn't do it, I was 'broken'.
"People need to know that postnatal depression is common. It's debilitating. We need more support."
MCAGNZ is calling for support for the petition which can be found at: https://our.actionstation.org.nz/p/fundpnd