Secondary school teachers warn that the ongoing teacher shortage is set to worsen as recruitment declines.

A recent survey revealed 16 percent of principals had to cancel classes due to dwindling staff, and the sector is relying on foreign-trained teachers and teachers over the age of 65.

Reports also show the number of secondary teaching students has halved since 2010.

Secondary Principals' Association President Vaughan Couillault says recruitment has gotten harder for specialised educational fields, such as maths and physics.

"The unfortunate thing is, the nature of our local supply is such that we're basically stealing off each other. Some teachers move over here, this one's moving over there, they're travelling, they're moving out of Auckland."

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