The government has announced changes to the bill reintroducing charter schools just two days before submissions on the legislation close.
The changes limit teachers' employment bargaining rights and give the publicly-funded private schools access to resources available to state schools.
Submissions to the bill were due to close on Thursday night but RNZ understands the deadline could be extended.
Associate minister of education David Seymour said the late changes were necessary to stop teacher unions from hamstringing the schools.
He said Cabinet had agreed to progress an amendment stopping unions from initiating multi-employer collective agreement bargaining for charter school staff.
"Unions will still be able to engage in single-employer collective agreement bargaining and charter schools will have independence to negotiate changes to employment terms and conditions relevant to their school," Seymour said.
He said the change was necessary because multi-employer agreements would affect charter school sponsors' influence over the terms and conditions offered to their employees "and could therefore influence the flexibility the schools can provide in teaching and other aspects of school operations".
Seymour said the amendment would also give charter schools access to the same teaching resources as state schools including specialist services like resource teachers and technology education.
He said the late changes were needed to counter expected union opposition to the schools.
"The government has had to assume the unions would encourage state school teachers to refuse to teach clustered services such as technology class to charter school children, and would initiate bargaining across the charter schools, thus undermining the employment flexibility critical to the model," he said.
Seymour said the unions had been running an aggressive campaign against the schools.
Post Primary Teachers Association president Chris Abercrombie said the government was desperately trying to patch up a model that clearly did not work.
"Cabinet's decision to allow charter schools to have access to the same level of teaching resources as state schools, is an acknowledgement that the charter school model of state funded private schools is unworkable," he said.
"Charter schools are being sold on a model of bespoke, bulk funded, flexibility. However, as we know, this is not possible, especially when it comes to supporting students with additional needs. By allowing charter schools to have access to state specialist services such as resource teachers, they are admitting that the charter school system can't work and that it needs to be able to cannibalise off the public functions of state education to provide some semblance of effective support for students.
"It increasingly looks like things are being made up as this process is rushed along, to shore up failings in the model and to protect private sponsors from the actual costs of meeting student needs, instead passing these costs back to the state. This will inevitably mean less funding for the needs of students in the state school system."
Abercrombie said blocking multi-employer collective agreements was a significant departure from employment law.
"We will be very interested to see what the International Labor Organisation, for example, thinks of this in relation to teachers' freedom to negotiate and to the ILO conventions to which the New Zealand government is a signatory," he said.
"No school is an island - evidence shows that schools do better when they don't compete. Having core terms and conditions that are consistent across charter schools would be a good thing, not a limitation.
Abercrombie said last minute changes showed the weaknesses of trying to introduce massive structural changes to the education sector under urgency.
Educational Institute Te Riu Roa president Mark Potter said the government's motivation for limiting teachers' ability to bargain collectively was worrying.
"A bad idea like charter schools isn't improved by adding more bad ideas like weakening people's fundamental rights to come together to win the pay and condition they deserve," he said.