Auckland school principals say a teacher shortage in the city is affecting the supply of relief teachers.
Schools have already said they were having trouble finding good teachers to fill their vacancies, but now they were also worried about finding relievers to fill in when teachers were unwell or on training courses.
Auckland Primary Principals Association spokesperson Frances Nelson said the pool of relievers dried up early last year and principals were not expecting it to be any better this year.
"Many of those relieving teachers have been shanghai-ed, if you like, into schools because of the desperate situation that is in front of principals," she said.
"But it does leave a serious shortage of relieving teachers."
The shortage was so bad, schools were having to use relievers who did not have the right expertise for the classes they were teaching, Ms Nelson said.
"I've heard stories of primary schools with a special needs unit being delivered a secondary teacher who has never taught in a primary school and certainly special needs isn't their area, and that is all that the agencies could supply on the day. It's not an unusual story."
The shortage would discourage schools from releasing teachers for training courses, and it might also discourage some from joining the government's flagship Communities of Learning programme, which required schools to let their best teachers go and advise other schools, she said.
Relieving agency Oasis Education said this year would be especially difficult.
"We struggled to find enough relievers for our client schools last year and I suspect that there will be a shortage of relief teachers," director Martin Strang said.
"The pool of teachers who want to do relief teaching is dropping as the teaching stock grows older and people retire and people leave Auckland."
A new requirement that relievers complete a training course in order to keep their registration was also reducing the number of available relief teachers because some teachers were not bothering to do the course, he said.
Auckland Secondary Principals Association head James Thomas said schools had been able to find relievers but it was getting harder, partly because schools had hired many of their usual relievers for short-term positions.
"We've got enough teachers in front of the kids, which is essential, but then when there are trips or illness or a range of things which need relief teachers, we don't have the same size pool of folk we know who are available in our communities."
Education Ministry associate deputy secretary Pauline Cleaver said its figures showed there were "slightly fewer" teachers working as relievers.
"That is because teachers that were working as relievers are choosing to take more fixed term or permanent roles," she said.
The flip side was that people choosing to work as relievers were finding more opportunities, she said.
That was borne out by an increase in the number of days relievers were working each school year.
The situation with relief teachers was no barrier to schools joining Communities of Learning, she said.
"We don't see any reason why this should deter schools from participating... In fact, one of the many benefits of joining ... would be allowing schools to develop and share relievers."
The Ministry was working hard to increase the supply of teachers and as numbers increased, it was likely more teachers would be available to relieve, Ms Cleaver said.