"There's a lot of culture involved and someone's ruined it for other people."
Longson had previously noticed that the fern looked like it had been loosely screwed on to the rest of the carving and could be easily detached.
"I was expecting it to happen," he said.
Auckland Council owns the carving which was commissioned as part of a regional public art programme.
Auckland Council arts & culture manager Richard McWha said, "We are aware of the damage to the carving.
"It was reported and the artwork was assessed by the artist who is currently undertaking the repair."
"We have a repairs and maintenance programme to respond to such issues with artists to anticipate potential vulnerabilities of artworks and design remedial works to lessen the occurrence in the future", McWha said.