The education source said it was not the first time the crack police team had been called on to foil attempts by students to dupe markers into giving higher grades.
Neither the New Zealand Qualifications Authority nor Police HQ would confirm how many questionable student scripts had been scrutinised in recent years.
"The Document Examination Section does not comment on the outcome of individual forensic examinations," a police spokesman said.
It is understood in the most recent incident a marker raised questions over whether the original exam had been altered between the time the student received the graded paper in January and when it was sent back for review a month later.
Students who sit NCEA are sent their marked exam papers in January, and if not satisfied with the result, they are able to have their work reconsidered.
NZQA investigated 299 students for suspected breaches during last year's external exams. It found 280 had cheated, while 18 had been cleared of wrongdoing. One other incident was still being looked at.
Of the breaches, 116 involved cellphones, 58 were caught with having notes, 19 were found to have other "unauthorised material" and 23 involved students being sanctioned for interacting with others.
The cellphone cases featured seven instances of students using their phones during exams, including communicating with others and accessing information.
Another 109 students had broken rules by having phones on them.
Twelve months earlier, just one student had been busted using a cellphone during NCEA tests, while 85 students were sanctioned for taking a phone into their exam.
NZQA acting divisional manager of assessment Sue Chalmers said exam centre managers were trained to ensure exams were run fairly. In the most serious cases students did not receive NCEA credits. But the NZQA would not say how many students had been stripped of grades.
Secondary Principals' Association of New Zealand president Sandy Pasley said pressure was ever-growing on students to score highly to get into top universities, and that led to some students cheating.
The NZQA had security measures to counter deception such as students showing photo ID to supervisors as they entered examination centres and banning cellphones.
Many schools were mirroring the strict examination protocol of end-of-year external exams for internal assessments.
"At the school I'm in, we have rules so the students know that if they've got a cellphone that's out; they immediately lose the credit," she said.
