A giant penis has appeared on the field of an Auckland school, and it's a mystery as to who committed the act of vandalism.

A member of the public spotted the penis on Sunday at Birkenhead College and posted it to Facebook.

"Is that a spade or what I thought it was?" they asked members of the Beach Haven and Birkdale Community group.

Some members speculated that it was an upside down Minnie Mouse or a rocket ship, and another member started the hashtag #whodrewthedicks.

It comes amidst the end of year prank season at New Zealand high schools.

Newshub.