The number of schools on the Qualifications Authority's watchlist for problems with NCEA assessments has trebled in two years.
The authority's annual report says 16 schools were on a one-year review cycle in 2015-16, up from 11 schools the previous financial year, and five the year before that.
Most schools are on a three- or four-year review cycle.
The authority says when problems arise with assessments in the National Certificate of Educational Achievement, they generally involve a risk to the credibility of results.
It says just 14 schools are on a one-year cycle right now, and none have lost their right to conduct NCEA assessment or had conditions placed on them.