An Oamaru teenager handed himself into the police on Tuesday, claiming he was responsible for throwing the Friendly Bay Playground train into the Oamaru Harbour.
The train, designed for children to play on in the Friendly Bay Playground, had been installed just three weeks before being found in the water on October 7.
The Oamaru Steam and Rail Society spent up to 120 hours constructing the train which is believed to have been thrown into the harbour on October 5.
Acting Sergeant Ross Lory said it was unlikely the 17-year-old male who had presented himself to them was alone in the vandalism.
"I'm guessing he had a few assistants, but they havn't been forthcoming at this stage," Mr Lory said.
He gave no particular reason for heaving the half-tonne piece of playground equipment into the water.
"He's a good kid. He'd obviously been drinking and just playing hijinks," he said.
The youth has volunteered his services to assist in the restoration of the train, which sustained damage after being thrown into the harbour.
"He'll be helping with the sanding and repainting, much to the delight of Bruce [Cawley]," he said.