The Post Primary Teachers Association says thousands more students could fail under the government's plan to replace NCEA with a new qualification.

It also warns the change will cost schools up to $58 million in incidental costs such as training and extra staffing.

The union said analysis by consultants BERL found 32% of the Year 12 students who achieved NCEA level 2 last year did so without "completing" more than one or two full subjects by achieving more than 14 credits in each of those subjects.

PPTA president Chris Abercrombie said that group of 14,930 would be at risk of failing when the government introduced a new certificate in 2029 that required successful completion of three full subjects.

"Those sort of students, who didn't formally pass three subjects, they're the ones that will be most at risk of not achieving in the future and we know there's a significant number of those students," he said.

Asked if students would simply change their approach under a new system and attempt more assessments in each subject, Abercrombie said that was possible.

"This is the best information we have at the moment about the impact of those changes," he said.

"If there's better information out there, we should see it."

Abercrombie said it was not clear to the union that the government had though through the effects of the new qualification on students.

"We're moving from a flexible system that's there to meet the needs of the students to a more rigid system that has the potential to put at risk more students leaving without a qualification," he said.

Analysis of submissions on the government's qualification proposal found 60% of educators who made submissions agreed most students should be able to pass four subjects.

Abercrombie said BERL also estimated the costs of introducing the new qualification would total $49-58m across 487 schools.

"The model includes the cost of teacher release time required for planning, implementation and curriculum-related activities, leadership time associated with managing the transition, and the purchase or replacement of learning resources and digital licences," the BERL report said.

It said the changes could cost large schools up to $260,000 each and small schools up to $34,400.

Abercrombie said there had been no clear information from the government about financial support for the changes.

He said opinion in schools on the changes was split, but there was widespread concern about the timeline and process for introducing the new qualification.

"There are some schools and some teachers and principals that are quite supportive of the change, some are very wary of the change, but actually what the most consistent concern is is what's the implementation plan," he said.

The Education Ministry said it had not seen the methodology behind the BERL estimate and could not verify it.

But it said this year's government Budget 26 included more than $80m to develop curriculum resources for all secondary subjects and fund professional learning and development for teachers to deliver the refreshed senior secondary curriculum and new national secondary qualifications.

"Ongoing curriculum support will also be available for secondary teachers and kaiako through the Ministry-funded Networks of Expertise (NEX) and Kahu Pūtoi," it said.

The ministry said it estimated how the qualification changes might affect student achievement using current NCEA achievement data and predictions about how students, schools and teachers might respond to the new qualification requirements.

"These estimations found that there could be a decrease in achievement rates with the introduction of the new qualifications. It is common to see a dip in achievement rates with a change like this, and is also seen in other countries," it said.

"However, the policy decisions made by Cabinet about the new qualifications took this into account and mitigations have been put in place to support student success. The Government has introduced a range of implementation supports to help mitigate any potential impacts on student achievement."

The ministry said mitigations included phased implementations from 2028 to 2030 so students would not switch between the old and new qualification systems during their time at school, training for teachers, and support and guidance including subject descriptors and assessment design support.

It said technical advisory groups and targeted engagement with schools would identify issues early before students were assessed under the new system.

The ministry said the first cohort affected would be the current Year 9 cohort, "providing several years for schools, teachers, and students to prepare".