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.
Two weeks later, the school told Ms Roberts that it had decided not to share its chemistry lab with Te Kura Hourua, saying the issue had become a distraction from its teaching.
Ms Hutt complained to Education Minister Hekia Parata, saying that she had offered resources to the charter school because she saw it as "an opportunity to support a highly successful emerging school in our area" and "to improve the chemistry results for Maori students".
"I need to advise you that this has become a 'major issue' requiring the executive of the PPTA to arrive unannounced in our school for a 'vital' meeting with staff," she said.
"My sole purpose is to improve education outcomes for all our students. I simply cannot allow any distractions to this purpose.
"I regretfully inform you that I need to withdraw Kamo High School support in terms of the use of the fume cupboard to allow the legal and safe mixing of chemicals for use of Charter School students."
Mr Seymour, whose party led the introduction of charter schools in New Zealand, said the PPTA had bullied the Kamo school out of a "win-win" arrangement.
The union had used the pretence of safety laws to force the school to end the deal, he said.
"It's just a way of blackmailing the school, bullying the school, because they're ideologically opposed to a charter school."
Ms Roberts said Te Kura Hourua should be able to afford its own chemistry facilities or lease them from a commercial provider.
The charter school received funding equivalent to $28,000 per student, compared to $15,000 per student for a similar-sized state school, she said.