Police were called to the clubrooms in John Wilson Ocean Dr yesterday morning, following reports vandals had smashed 14 windows overnight.
Senior lifeguard Bryan O'Brien (18) said more than $10,000 damage had been caused to the building, including many windows which were replaced last year after another attack.
The building was insured.
"This is gut-wrenching," he said while surveying the scene.
The damage was a setback for the club, with the season set to start on October 30.
Constable Jake Milne, of South Dunedin, said a metal petrol cap, which was found at the scene, was used to smash the windows.
Attempts to find fingerprints at the scene were unsuccessful, he said.
The clubrooms' isolated location had made it the target of vandals in the past, and it was disappointing people would commit such mindless acts.
Police believe the vandalism was not connected to a Monday night incident, when windows in two businesses in South Rd and one in Hillside Rd were smashed by small metal balls.
Anyone with information was urged to contact South Dunedin Police on 455-1199.
