Experienced relief teachers are opting out of the profession because they have to update their training or get full teacher registration.
The Education Council has required teachers who do not have full registration to complete a $4000 Teacher Education Refresh course every six years if they want to keep working.
But relievers in their 60s told RNZ the course was irrelevant for people who did not want a permanent teaching job, and the rules ignored their decades of experience in the classroom.
Kerikeri teacher Sue Evans, 64, said she taught in England for 15 years before moving to New Zealand in 2008, and since then she had been relieving for her local secondary school.
She said her work in New Zealand included long-term relieving positions and more recently she had been working three or four days a week as a reliever.
But Mrs Evans said that work had stopped because she did not have full New Zealand teacher registration and she did not want to do a $4000 Teacher Education Refresh course.
She said she did not need an update on modern teaching methods because her local school gave her plenty of training while she was working as a long-term reliever.
"If I had only done relief say one day a week in different subjects, I wouldn't be aware of it. But because I've done a lot of long-term relief you also do in-school training," she said.
"There's a shortage of teachers and yet they're not taking into consideration people like me who've got a lot of experience and done a lot of work here."
A 68-year-old teacher - who asked not to be named - told RNZ that he was doing the refresh course because he enjoyed relief teaching and did not want to stop.
But the man said the course was not relevant for people who wanted to do only relief teaching.
"As a relief teacher I'm not learning anything of value, all that we're doing is theory," he said.
"As a teacher you want the practical aspects of being able to manage and control a class and try and make sure they're as productive as possible. Having all the theory about pedagogy and teaching by inquiry and things like this, it's not really relevant."
The man said he had been relieving for the past 17 years and teachers with a lot of experience should not have to do the course.
"It's crazy and there's a teacher shortage and they're trying to think of ways to get new teachers - well why don't they just retain the old, experienced teachers who have got all these years of experience and are doing a good job, that's what I can't understand."
The deputy chief executive of the Education Council, Lesley Hoskin, said from this year teachers have been able to do a Ministry of Education funded in-school induction and mentoring programme instead of the refresh course.
She said the in-school programme was good for relievers and took account of their classroom experience.
However, Mrs Hoskin said the refresh course and the induction programme aimed to ensure teachers, including relief teachers, were fully certified.
"To ensure that we've got high quality teachers in front of our students all day, every day, so whether you're relief teaching, a part-time teacher, or a full-time teacher, the premise of both of those programmes is to support all teachers on provisional certificates to move towards their full practising certificate."
She said about 300 teachers a year needed to do a refresh course or in-school programme if they wanted to continue teaching.