New Zealand's biggest high school has been hit by $20,000 worth of damage after embarrassing images of giant male genitals were burned into the playing fields.

Rangitoto College principal David Hodge said the damage was done at the end of October when Year 13 students were departing on study leave.

But the offending images did not appear until later, when the powerful chemical started to kill off sections of the turf.

"If it was meant to be a joke, it is just not funny."

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The 3000-roll college - which last year paid $80,219 to remedy vandalism - already spends about $15,000 a year maintaining the wicket.