Figures released by the Transport Agency show fewer than half of those sitting the new restricted licence test are passing.
The test, which requires learner drivers to complete at least 120 hours of supervised practice before they can sit their exam, came into force in February.
In March, 38% of learner drivers who sat the test passed; last month that figure increased to 46%.
The pass rate under the old system was 80%.
Transport Agency chief executive Geoff Dangerfield says a significant drop in pass rates was expected when the new testing regime was introduced.
He says the test was designed to improve road safety and reduce the number of deaths and injuries on the country's roads.
New Zealand has the highest road death rate for 16 to 17-year-olds among all developed countries.