The Post Primary Teachers Association (PPTA) has called for state schools to stop offering alternatives to the NCEA.
The union's president, Angela Roberts, said foreign qualifications such as the Cambridge International Exams and the International Baccalaureate were undermining the national secondary school qualification.
She said schools were offering them purely for marketing purposes and they made the public think the NCEA was not up to scratch.
"What you're implying is that NCEA isn't the world-class qualification that it is," she said.
Ms Roberts said teachers, schools and successive governments have invested a lot of time and money into NCEA.
"It's diverse, it's holistic, it's robust and it is built on a New Zealand ethos and a New Zealand world view - it is world class. I don't see why we need to spend more money on offering an alternative."
The headmaster of Auckland Grammar School has dismissed the PPTA's suggestions
Tim O'Connor said his school provided Cambridge exams for their academic rigour.
He said the system suited many of the school's students because it was exam-based, but it was not "better" than the NCEA, which Grammar also offers.
However, another big Auckland boys' school, Westlake Boys' High, is phasing out Cambridge from next year because NCEA has improved over the past 10 years.
So what are the various qualifications on offer and how do they differ from NCEA?
The National Certificate of Educational Achievement was introduced in 2002.
It divides each subject into standards that measure particular capabilities. For example, a Level 1 maths standard is "Apply measurement in solving problems". If students can do that, they "achieve" the standard and the three credits it is worth. If they do it well, they can get a merit or excellence grade as well.
The standards are assessed internally throughout the year with essays, projects and internal exams, and at the end of the year in externally-assessed exams.
Each standard provides credits and students need a minimum number of credits in order to get an NCEA certificate. The certificate is provided at three levels - 1, 2 and 3.
NCEA is recognised for entrance to foreign universities.
The main difference between NCEA and previous New Zealand qualification systems is that it seeks to express what students can do, rather than ranking them against one another. Students generally do not attempt every standard in a subject.
The Cambridge exams are provided by Cambridge International Examinations, part of Cambridge Assessment, which in turn is part of the University of Cambridge in the United Kingdom.
The qualifications it offers secondary schools are the International GCSE and International A and AS levels - based on the English school qualification system.
Students are tested in exams at the end of the year, though there are also end-of-year internal coursework assessments and practical tests.
The exams are offered in 160 countries, and where possible the curriculum is tailored to reflect the local context.
The Association of Cambridge Schools in New Zealand lists 64 member schools.
International Baccalaureate programmes are offered by 22 schools, including several private schools and some state primary and secondary schools.
The alternative to NCEA is the IB Diploma. Students work toward it over two years, and must complete exams and a 4000-word essay. There is also some internal assessment.
The qualification is offered by International Baccalaureate, which was set up in Switzerland in 1968. It charges schools about $16,000 a year to offer the programme.