Act Party leader David Seymour is accusing a teachers' union of bullying a state school which offered to share resources with a charter school.
Letters released by Mr Seymour show that the Post-Primary Teachers' Association (PPTA) urged Kamo High School in Northland not to offer support to charter school Te Kura Hourua o Whangarei Terenga Paraoa.
The Decile 5 school had agreed to give the charter school access to its chemistry laboratory.
In a letter sent on May 10, PPTA president Angela Roberts warned the school's principal Joanne Hutt that sharing facilities could go against health and safety laws and would upset teachers who had voted not to support charter schools.
"At the least this will lead to resentment and ill-feeling, and would be an unfortunate situation where there is no plausible benefit to the students or staff of Kamo High School," she said.