The teachers' union has hit out at Gilmore MP Ann Sudmalis' decision to email schools about the federal government's Independent Public Schools initiative.
NSW Teachers Federation regional organiser Nicole Calnan labelled the letter as "dishonest", claiming Ms Sudmalis had encouraged staff to opt out of the public system.
On Wednesday, Fairfax Media revealed that Ms Sudmalis had written to principals earlier this month, encouraging them to back the policy.
Ms Calnan said education professionals had felt the move was dishonest.
"Certainly that was what was expressed to me when I was contacted by principals and teachers who had received that email," she said.
"They were wondering why she had contacted them about it, because they can't opt out of the NSW public education system and that's effectively what Ms Sudmalis was asking them to do."
Federal Education Minister Christopher Pyne has encouraged MPs to promote the policy in their area, despite the O'Farrell government ruling out its introduction in NSW.
The head of the NSW Education Department has since written to public school principals, warning them to ignore lobbying by federal politicians.
NSW Teachers Federation president Maurie Mulheron accused Mr Pyne of "mischief making".
Ms Sudmalis defended her email, saying the plan could improve outcomes for students.
"I'm a strong supporter of the Independent Public Schools program and the potential positive impact it will have on local schools," she said.
"I am eager to work with the NSW government on this important issue.
"If 'mischief making' is writing a letter, offering an opportunity for a school to work through their state Department of Education with their parents, their students and their local business associations to get better outcomes for their children, well, I can only say I'm stunned."
Ms Sudmalis said she had not been contacted by the Department of Education following the email.
NSW opposition spokesman for education and training Ryan Park urged Ms Sudmalis and her federal colleagues to stop "this carrying on".
"This reform is not welcome in NSW and the federal government has to stop trying to getting involved in NSW school issues," the Keira MP said.