Parents could increasingly find themselves being asked to pay just to apply to enrol their children at school.
Secondary school principals say the annual enrolment process is becoming more expensive for popular schools, which have to double-check people who say they live in-zone really do.
They say the Auditor-General has cleared them to ask for donations from families who apply to enrol from outside their zones - a practice the Ministry of Education currently says is unlawful.
Most schools did not ask parents to pay or make a donation to cover the cost of enrolment.
Those that did included Waikato Diocesan School for Girls, which had a $40 administration charge on applications to enrol, Sacred Heart and St Peter's Colleges in Auckland, which asked for $100 donations to cover enrolment costs, and Auckland Grammar, which requested a $50 donation.
Ministry of Education guidelines said schools could not charge fees for enrolment interviews, for enrolment applications or as a condition of enrolment.
The ministry said schools could ask for a donation as long as it was clear that was voluntary.
Secondary Principals Association president Sandy Pasley said the process had become more expensive because schools had to employ people to physically check enrolment details were correct.
"They can be a really expensive business, particularly for those schools that have enrolment schemes," she said.
"There's a lot of checking that has to go on that people are actually eligible to apply for enrolment within certain enrolment criteria, and that's not an easy exercise to do and it's not a cheap exercise."
Ms Pasley said schools should be able to cover their costs.
"Schools need to get some recompense for the expense that's being created and whether it's a charge - I know there's some talk of schools asking for a donation - but certainly they're out of pocket, and something needs to happen to help those schools."
Ms Pasley said schools also needed clarity about whether they could ask for donations from families applying for enrolment from outside of a school's zone.
Ministry of Education spokesperson Katrina Casey said that was not allowed.
"We've recently had legal advice that because out-of-zone parents are likely to be under the impression that an application for pre-enrolment will not be processed unless the donation is paid, the request for a donation has the same operative effect as a compulsory fee for pre-enrolment," she said.
"As a result of the legal advice, our position is that such a request is unlawful."
But Mount Albert Grammar School said the Auditor-General had investigated and approved its request for a $30 donation for out-of-zone applications.
Auckland Grammar School principal Tim O'Connor said the Auditor-General had also looked into his school's practice of asking for a $50 donation for both in-zone and out-of-zone applications.
"There's nothing unlawful about our action," he said.
"The key thing that we've been told is to ensure we take it off a checklist to suggest that it could be something that is compulsory, so we have changed that process to make sure that we are fully compliant."
Mr O'Connor said about 80 percent of families paid the enrolment donation but it only covered a small amount of the cost of enrolments.
"The $50 donation that we receive covers about 30 percent of our costs overall," he said.
Mr O'Connor said the school could get 300-400 applications for only about 30 out-of-zone places.
The principal of Cornwall District School in Auckland, Janine Irvine, said without a financial contribution out-of-zone places could be more trouble than they were worth.
"You could get potentially quite a large number of applications and of course that's going to impact on the amount of hours your administration staff have to deal with them" she said.
"And so I suppose our board of trustees always weighs that up, to see whether it's actually viable to offer out-of-zone places."