Figures from the Department of Corrections show more than a third of people sentenced to community work in the past year failed to complete it.
Community work is a court-imposed sentence that requires offenders to work unpaid for non-profit organisations. It involves tasks including painting, gardening and recycling.
The department's figures show that in the last financial year, almost 20,000 offenders did not complete their community work.
It also shows that while in earlier years completion rates have been about 75%, in the year to July that dropped to 64%.
The department says a sharp increase in the number of people on community work sentences and a lack of resources is responsible for the increase in the number failing to complete their sentences.
A spokesperson for the department, Katrina Casey, says staff struggled to cope with the increase in the number of people being given community sentences.
However, Ms Casey says with extra money in the budget, that has now been rectified.
Corrections Minister Judith Collins has asked the department to explain itself, and will meet with officials on Monday.
Ms Collins says she also wants to know what happens to people who fail to fulfill their sentences.
