The PPTA says the legal action came only after repeated efforts to work with the Ministry of Education on a solution weren't taken seriously.
The case has caused some division within the union. The Herald understands the PPTA Onehunga High School branch approved a motion supporting the Teach First programme, and objecting to the PPTA's legal action.
The branch chair could not be reached for comment, and principal Deidre Shea referred questions on Teach First to the ministry.
PPTA president Angela Roberts said she was aware of some concerns and could understand why some members felt the union had been "a bit heavy-handed".
However, Ms Roberts said the action came only after repeated attempts to engage with the Ministry of Education were unsuccessful, with suggested solutions including making Teach First positions for trainees, not teachers.
The issue was not with the Teach First programme, as much as upholding its collective agreement and employment law, Ms Roberts said.
"I have had conversations with members from various branches, and once we get an opportunity to clarify that this isn't about the programme but about the employment relationship, people start to understand that if this is something to be bedded into the landscape, then it needs to be done right."
A spokesman for Ms Parata said the ministry was dealing with the issue at this stage.
Lisa Rodgers, the ministry's head of early learning and student achievement, said in a statement that its "door remains open" to the PPTA - but it expected Teach First students to continue with their training.
"We are carefully considering the Employment Relations Authority decision and seeking further advice before determining our next steps."
Teach First
• Takes high-achieving university graduates for its on-the-job training programme.
• Begins with an eight-week intensive residential programme.
• Graduates then move in to low-decile schools. They are paid a full-time salary but have a maximum of 12 hours a week contact time with students. Mentoring is ongoing.
• Had 300 applications for its 20 places for 2016.