New research from Auckland University shows only one in five toddlers living in the most at-risk families are getting access to social support services in the first 1000 days of their lives.
The Growing Up in New Zealand project, which tracks 7000 children from before their birth onwards, has found those living in the poorest or most vulnerable families are more likely to suffer health and behavioural problems.
Associate Professor and project head Susan Morton said most of the families potentially in need of support were not getting it.
"There seems to be some sort of gap between what is intended in terms of support for those family most in need, and how we actually get those services to reach those families."
She said vulnerable children could be missing out because their families moved around frequently, or because of their parents' reluctance to seek help.
"There are also issues from the families' point of view in terms of how acceptable and accessible those services are for them, in terms of meeting their needs."
Researchers said the traditional approach of targeting services to deprived areas might not be working and, to get help to children who need it, services may instead need to focus on several risk factors.