Two charter school owners say the government should give the publicly-funded private schools more financial assistance to help them grow.
The Villa Education Trust and He Puna Marama Trust say the schools' property funding does not stretch to building or fitting out new classrooms to keep up with enrolment increases.
Both trusts own growing charter schools and the Villa Education Trust wants extra funding for growth while He Puna Marama Trust is seeking access to a government loan.
Their call for more funding is likely to infuriate charter school critics, who say the schools shouldn't get any government money at all.
The eight charter schools get funding specifically tagged for property and their financial statements showed most did not use all of it on property expenses last year.
Alwyn Poole from the Villa Education Trust which owns two Auckland charter schools said the money did not have to be used on property but even if it was, it was not enough to pay for expansion.
"We get cashed up funding and the ministry, I guess for convenience, labels it for this, for this, for that, but there's no expectation that we actually spend it on those things," he said.
"But even within that calculation, it's more for ongoing property [costs], it's not for an expansion."
Mr Poole said the government spent millions of dollars expanding state schools and while he was not asking for that much money, the system should be more equitable.
"It's maybe time to step back and have a little bit of a look and say 'well, what happens when these schools are successful and people want to come to them'."
Mr Poole said the Villa Education Trust had asked for extra funding when it applied last year for permission to grow one of its schools, the South Auckland Middle School to 180 students.
He said the growth was approved, but there was no extra money.
The under-secretary for education, the ACT Party leader David Seymour, said growth was a good problem to have, but there would not be any more funding for the schools.
"They can use their annual grant to rent any property they like, and they may in some cases save money on property and put more money into teachers and curriculum and so on, but we're not going to start giving capital grants to schools as well as a per-student amount each year to cover their property needs."
Mr Seymour said the charter school funding formula ensured the schools received the same funding per-student as a state school and he could not see the government changing that.
However, He Puna Marama Trust chief executive Raewyn Tipene said it was looking for a loan from the government rather than a special grant or increased funding to cover the costs of growth.
The trust owned two charter schools in Whangarei, a secondary school Te Kura Hourua o Whangarei Terenga Paraoa and a primary school, Te Kāpehu Whetū.
Ms Tipene said the trust had already spent $5 million on property but it needed more.
"For us to grow to the numbers that we would love to grow to, we need more support from government by way of a cost-neutral capital arrangement with them. A mechanism for accessing some funds that we would ultimately have to pay back."
Meanwhile, the call for extra government funding for South Auckland Middle School in Manurewa did not impress local school principals.
Manurewa Principals Association president Karen McMurray said charter schools plainly received more than enough funding because they were able to pay for things that state schools could not, such as uniforms and exercise books.
She said the funding for charter schools would be better spent on state schools.
"I would like to see the money being put into our state schools so that we can continue to upgrade those and develop the quality programmes that we would like to see for our children."
Ms McMurray said charter schools were likely to run into trouble with under-maintained buildings if they did not use their property funding for property expenses.