Clutha District Council district assets manager Jules Witt said vandals targeted the toilets near the northern end of the Balclutha bridge ''fairly regularly'' at weekends during the past three months.
The most recent incident happened two weeks ago.
In total, about $3600 of vandalism repair and clean-up expenses had been incurred, through repeated damage to toilet paper holders and paper strewn across nearby streets and footpaths.
He said this had not only created an unpleasant mess but, along with the damage caused, was expensive to clean up.
''These unnecessary costs are ultimately borne by local ratepayers and at this point in time the best way to put a stop to it is to lock the toilets during the hours of darkness.''
Locks were installed yesterday.
Other public toilets throughout the district were often targeted by vandals.
Mr Witt said the council would prosecute anyone found causing wilful damage. Anyone who saw this type of anti-social behaviour was urged to contact the council or police in order to prevent these unnecessary costs, he said.
