The Ministry of Education is questioning the reliability of the NCEA system, following an internal review of exam results.
A report shows students whose tests are marked by their own teachers are up to three times more likely to get top grades.
Twenty-eight percent of internally assessed papers at Level 3 are being judged as "excellent".
While only 11 percent of students marked by external assessors perform that well.
Research fellow at thinktank NZ Initiative Briar Lipson says NCEA results have always been plagued with inequity, but the new figures show the gap is widening.
"Teachers are faced with a moral dilemma when they are asked to be both the educator and independent assessor - which is a conflict of interest."
Ms Lipson says many teachers face pressure from parents and others to do internal marking, in order to have more control over the results.
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