The Tertiary Education Union wants polytechnics to take on more tutors to cope with increasing numbers of New Zealanders enrolling.
Many institutes made cuts because of a fall in foreign student enrollments during the Covid-19 pandemic, union president Tina Smith said.
But domestic student numbers were up by about 22 percent at several polytechnics, she said.
"Sadly I'm not hearing of any increases in staff by 22 percent. What I've heard about is that staff who have left have not been replaced - they've been really slow on any recruiting."
Smith said tertiary institutions needed a better funding system, so they were not so reliant on foreign students - who paid higher fees than domestic ones.
The quality of education would suffer if staff were over-extended, she said.