Patients are being charged up to $250 for seeing a doctor outside surgery hours.
A nationwide survey by the Ministry of Health shows a home visit in the region can cost $400.
More than 40 clinics around the country were charging at least $46 for each out-of-hours consultation for a school-age child and more than $65 for an adult.
Ten clinics charged $66 or more for school-age children and $41 or more for children under six.
One unnamed clinic charged adults $250, although the report says that charge was rarely implemented.
Nine others charged adults at least $86 for consultations outside surgery hours.
The ministry report from October last year, obtained under the Official Information Act by the National Party, says district health boards are having mixed levels of success in lowering after-hours fees.
The figures may prompt the ministry to impose restrictions on what GPs can charge.
Ministry spokesperson Jim Primrose says it is concerned at some of the fees, even though New Zealand does well by international standards.
Dr Primrose says the report asks whether a mechanism should be introduced to control out-of-hours charges, in the same way as daytime fees are controlled.
The Child Poverty Action Group says children are being admitted to hospital because families cannot afford after-hours GP fees.
Spokesperson Nikki Turner, herself a doctor, says it did its own survey last year showing fees of up to $120 for an after-hours visit for a child.
Dr Turner says costs can be prohibitive for some families and some children are ending up in hospital later on as a result.