Private schools hope the Government will increase their funding and index it to that of state schools.
The Government is considering advice about subsidies for private schools and teacher unions fear it will increase them.
Radio New Zealand's education correspondent said Government funding for private schools is capped at $45 million per year, regardless of the number of students enrolled.
The Ministry of Education says the funding is currently a quarter of that of state schools on a per student basis, down from a third 13 years ago.
Independent Schools of New Zealand represents about half the country's 85 private schools. Executive director Deborah James said the Government could increase the percentage and keep it constant.
She said the Government gets more in GST on private schools' fees than it spends on their subsidies, and every enrolment in a private school saves it money.
Ms James said the Government could raise its funding to 45% of that of state schools and it would still be better off.
More funding feared by unions
Unions representing primary and secondary school employees are worried the Government is preparing to increase its funding for private schools.
Education Minister Hekia Parata is refusing to release advice she has received on the subject, but says the information is under active consideration.
Radio New Zealand's education correspondent says that has alarmed the Post Primary Teachers Association and the Educational Institute because the Budget is only a month away.
They say private schools should not get any more public money.
But Independent Schools of New Zealand, which represents half the 85 private schools in New Zealand, says the schools need more help.