Teacher unions have written to aspiring charter school operators in a last-ditch attempt to warn them off setting up the schools.
Radio New Zealand's education correspondent said the letters are the first time the Post Primary Teachers Association and the Educational Institute have directly lobbied groups wanting to set up the schools, which the government is calling partnership schools.
The letters say the schools could be cancelled as early as November next year with no compensation because opposition parties have vowed to abolish them.
They are being sent to 35 organisations that expressed interest in the schools, four of which are known to have made it to negotiations with the Government.
PPTA president Angela Roberts said charter schools will damage nearby state schools.
C-Me Mentoring Foundation Trust chief executive John Kotoisuva said he is not bothered by the letters.
Mr Kotoisuva said he does not agree with the unions, because the current school system is not meeting the needs of many Pasifica children.