The Education Review Office is questioning whether the Government is getting good value from the $200 million spent each year on developing teachers' skills.
The schools watchdog is releasing two reports, which it says should be a wake-up call for schools.
It examined how professional development was carried out for teachers in 317 primary schools and 44 secondary schools.
It found that 80% of primary schools managed such training either well or very well but in secondary schools more than 40% needed to do better.
Chief reviewer Graham Stoop says it can be a complex matter, but school boards must still ensure the considerable amount of education funding for teachers' professional development is spent well.
Mr Stoop says it is important school boards understand the impact of this training, noting that there is a clear link between good professional development and student achievement.