The government's pledge to bring in 400 teachers from overseas for next year will not solve the shortage, says the secondary school teachers' union.
Post Primary Teachers' Association president Jack Boyle told Morning Report at least 700 teachers are needed.
"If you don't worry about the fact that one in five secondary teachers are older than 60 and one in ten is older than 65 - if you just assume they're gonna keep on keeping on - if you get 400 in areas where there's nobody, that might be papering over some of the cracks while we build something more sustainable."
"But [we need] something in the order of 700 plus per year, as a baseline."
Mr Boyle said while it was good the government acknowledged the shortage, bringing in migrant teachers was not the answer.
"Are these people going to have preparation for teaching in New Zealand with a culturally responsive pedagogy, and to be quite frank, with international teacher shortages, are they even out there?"
"Paying recruitment agents as they have been for New Zealand-based offshore [teachers], which so many principals have had to use, the numbers aren't always that great."
He said making the industry more appealing by raising wages would be a better way to fix the problem.
"In terms of the sustainability... instead of putting the hard work into making teaching a sustainable, rewarding and valued career in New Zealand for New Zealanders, we're going to see if we can find what we need offshore and just hope that we get by and that's what the risk is.
"We absolutely need to get the pay and conditions right to hold onto those great teachers that we've already got and to [incentivise] New Zealanders who are going through their tertiary pathway."