Public Utilities General Manager David Wright said that an increase in mistaken bills was traced to two meter readers.
Investigators put them under surveillance and found that the workers spent much of their time sitting in their trucks, plugging in phony numbers.
Wright said he couldn't give out names or other personal details because personnel matters are confidential.
Hundreds of customers were affected. The utility says their bills will be automatically corrected next time their meters are read.
The Associated Press contributed to this story.