Labour says 22 of 26 practices in central Auckland aren't offering free doctor visits for under-13s.
Annette King estimates up to 5000 children are missing out, and she's challenging Health Minister Jonathan Coleman's claim that uptake has exceeded expectations.
Free doctor visits for all children under 13 started on July 1. Until then children under six were free.
At the time Dr Coleman said about 96 percent of practices around the country had signed up to free visits for under-13s.
He said he was "stunned" by the uptake level.
"The initial uptake for under-sixes was around 70 percent of general practices, that's now up to 98 percent," he said.
"We expect the number of general practices taking part in free under-13s will rise to a similar level."
Practices don't have to offer free visits.
If they do they're funded to cover them, but there's a debate around whether it's enough.
Ms King, Labour's health spokeswoman, says it's clearly an issue.
"This is another example of the Government under-funding a policy and expecting an already stretched frontline to pick up the pieces," she said.