The Tertiary Education Union says profits at polytechnics are too high and need to be reined in.
Overall, the polytechnic sector recorded a $90 million surplus last year and $70 million the year before.
Radio New Zealand's education correspondent says this is equivalent to about 8.3% and 6.9% of their income respectively.
The union says some institutions are making much more than the recommended range of 3% to 5% and the money should be spent on providing good education.
Polytechnics say they are seeking higher surpluses as a buffer from financial shocks.
The Tertiary Education Comission says it has no problem with any polytechnic's surplus and says 3% to 5% is the minimum expected.