A group of plumbing apprentices will have to resit their course because hundreds of test results were incorrectly reported as passes.

A New Zealand Qualifications Authority inquiry has found the Plumbing, Gasfitting, Drainlaying and Roofing Industry Training Organisation reported 339 "achieved" results for trainees when course provider Manukau Institute of Technology (MIT) reported only 81 passes.

Some results didn't match the unit standards they were supposed to cover and the industry training organisation (ITO) didn't supply requested assessment schedules to show the course had been taught and marked correctly.

"NZQA is concerned there is no certainty whether these trainees really achieved the plumbing and gasfitting unit standards they were credited with," the report says.

It orders the ITO to provide detailed assessment materials for future courses and won't accept results from the organisation until it can show it is assessing trainees properly.

However, the report rejects a claim by Garry Cruikshank, who taught the block courses at MIT, that the ITO deliberately falsified results.

Mr Cruickshank said MIT passed only 58 students and the ITO passed about 600 altogether. He said the report ignored the systematic nature of the misreporting, which had affected students on three other courses and at Wintec in Hamilton, which used the same course materials.

Mr Cruickshank has written to Tertiary Education Minister Steven Joyce calling for an independent investigation.