A large phallic symbol has been etched into the grass at Birkenhead College on the North Shore.

A photo of the image resembling a penis was posted on the Beach Haven and Birkdale Community Facebook by a resident who noticed it when she walked past the school this morning.

Tricia Tan said the drawing was up the stairs from the college's main carpark and in front of C Block. Tan was not sure of the exact size of the drawing, but said "it's really big".

The image appeared to have been etched into the grass using a chemical such as weed killer.

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Members of the community Facebook page have risen to the challenge, posting humorous remarks that it was a rocket ship.

"Haha been a few years since there's been a frank and beans at the school," another member commented.

Several others were certain the penis outline on the grass was down to an end-of-year prank by students.

The school could not be reached for comment.