Dr Coleman said the GPs which had opted into the scheme cared for 98 per cent of children aged under 13.
He said the initial uptake for under-sixes had been 70 per cent, and that had now risen to 98 per cent.
"We expect the number of general practices taking part in free under-13s will rise to a similar level."
Extending the zero-fees policy to under-13s will cost $90 million over three years, plus another $27.5 million over three years in ACC funding to cover accident-related visits.
This funding was expected to allow an additional 400,000 children to get free check-ups.
Children will be able to make unlimited visits at no charge, though funding for GPs is capped at two visits a year per child.
The funding level would be reviewed after a year to make sure it was covering doctors' costs.
Dr Coleman said since zero-fees visits for under-13s were introduced in Northland in October, 10 per cent fewer children in this age group had turned up in hospital emergency departments.
Asked about the 44 clinics which had opted out, he said: "The key thing is in all those locations where a practice hasn't opted in ... there's an easily accessible alternative."
Karori Medical Centre GP Jeff Lowe said the zero-fees policy was a good investment because it was cheaper to address health issues early in life.
He said 1 in 10 of his practice's 1400 patients were in the six to 12 age group.
"That means just over 1400 children will get free access to the advice and treatment that they need and the medical that the need to keep them well."
Father-of-seven Paul Retimanu, a patient at the medical centre, said three of his children would benefit from the policy.
He said that while cost was not an obstacle to treating his kids, he knew it was factor for many Maori and Pacific families.
"Everything's a saving. If I look at our kids, if they get sick twice through winter that's $40 a visit plus $5 to $10 for a prescription. Times that by how many children you have - that does get quite significant."
