NSW Primary Principals Association president Phil Seymour said he was concerned that if the updated policy was enforced "rigidly" that parents would "vote with their feet".

"They will say 'This is no longer the system for me' and will go to the private system - and that's not what we want to happen," Mr Seymour said.

Related: Hunter schools must have 'consistent' enrolment policies

As previously reported, schools are required to set an enrolment ceiling, for example 400, based on their number of permanent buildings.

The ceiling includes a buffer of places, for example 20, to accommodate local students enrolling throughout the year.

If they reach their before-buffer maximum, in this case 380, they are expected to turn away new out-of-zone students.

But the degree to which schools have complied with this arrangement varies.

Several are exceeding their enrolment ceilings, in some cases because of an influx of local enrolments.

Some principals have included demountables as permanent buildings in their calculations.

Some have made exceptions for certain out-of-zone students, such as younger siblings of current students.

Related: Biddabah Public School parents raise questions about out of zone enrolments | poll

Following the policy's July 22 implementation, schools have to comply with a centrally-set enrolment "cap", based on permanent accommodation. Principals will set the buffer.

Demountables will not be counted towards the cap unless new or replacement accommodation is under construction.

"No additional accommodation (permanent or demountable) will be provided to cater for increased enrolments resulting from non-local enrolments," the policy says.

Schools are now able to use a 100-point residential address check to determine students' entitlement to enrol.

Related: Are Hunter schools overcrowded?

The policy says schools won't consider new out-of-zone enrolments if they can't accommodate them below their buffer, which reflects many schools' current practices.

When demand from out-of-zone families exceeds the number of available places under the buffer, the school must develop criteria and establish a panel to make decisions on applications.

"Priority should be given to siblings of currently enrolled students, where possible," the policy says.

Related: Newcastle and Hunter schools facing population "crisis" as enrolments surge | interactive, video

In some cases, a principal will not be allowed to offer places to out-of-zone children without informing the principal of the applicant's local school and gaining department approval.

A Department of Education spokesperson said the policy would provide principals with "greater clarity when undertaking enrolments".

"Across the state, many communities are changing," the spokesperson said.

Related: Read more education news here.

"Some schools that used to be able to accept out-of-area enrolments no longer have the room to do so.

"Such schools will, of course, continue to enrol any in-area students.

"The enrolment of children at their local public school will continue to be guaranteed and appropriately accommodated.

"Schools with room to accommodate non-local enrolments will continue to do so.

"In schools that have the capacity to accept non-local enrolments, the enrolment of siblings will be accepted in most cases.

"Students currently enrolled in NSW public schools will not be affected.

"Support for students with disability will continue to be provided in the educational setting that best meets their needs."

The spokesperson said Rutherford, Thornton, Newcastle East, The Junction, Hamilton South, Biddabah, Belair, New Lambton and New Lambton South public schools weren't "expecting to be able to take any new out-of-zone enrolments in 2020".

Rutherford, the region's largest primary school, has 939 students. Its latest enrolment policy shows its ceiling is 594, including a buffer of 21 places.

Mr Seymour said the original policy had "not been enforced or applied" consistently across the state and he "understood the need to rationalise".

"We do have schools that are empire building and then there's pressure on the department to build more and more," he said.

"We can't keep building and building when some schools have empty rooms."

But he said he hoped after meeting with the department this week the policy would be amended.

Mr Seymour said he was "very concerned about sibling rights" and believed there should be a "blanket rule" that younger siblings of current out-of-zone students be allowed to attend.

"These are families we'd be splitting up," he said.

"You've got to be compassionate and there has to be consideration for the families.

"If a kid starts at a school the families should be able to continue there.

"My mum might live in an area and I drop two kids off before I go to work. That's helping families and we should be doing that."

He said many schools cared for children who were refugees, in out of home care or had turbulent home lives.

"School is often the most stable part of their lives," he said.

"Hopefully the department will see the light and it will be a common sense approach between the principal and director to talk that through."

He said there were some "patches", including around Hamilton South and Belair, where principals had previously been told directly "you are not to take any non-local enrolments".

"It caused them a great lot of angst - we don't want that."

Mr Seymour said the formula for determining the cap had to be reviewed so it was "true and correct".

"We've got schools that have had demountables for 10 or 20 years and they should be included in the cap," he said.

"These schools may have 400 students and 200 are being taught in demountables - and they're locals as well.

"When working out the cap you need to be looking at the buildings you've got and what are the particular needs at that school.

"You might think it makes sense to lift the cap, back fill and build the accommodation you need.

"Or if it's just a wave, to put demountables in for ten years."

The Federation of Parents and Citizens Associations of NSW said it is considering the changes.