The Teachers Council says up to 300 hundred teachers will have to update their training if they want to keep working next year.
Those who have not gained full teacher registration within six years of getting their teaching qualification must spend about $4,000 on further training, or leave the profession.
The teachers' regulatory body says some people have reached senior positions, such as deputy principal or head of department, with only provisional registration.
It says qualified but unregistered teachers are likely to have little teaching experience, and must complete courses that will bring them up-to-date with changes in the education system.