An inquiry has been launched at JobCentres across Surrey and Sussex after a client claimed he was able to overhear other users' security details.
JobCentres have been told to ensure details are secure
The customer, from Haywards Heath, West Sussex, gathered the names, addresses and National Insurance numbers of five people while at his local branch.
"I am concerned the information could be used for identity theft," he said.
JobCentre manager Lynn Connolly said: "This is clearly poor practice and I have instigated an immediate inquiry."
The complainant said he was able to gather the information while he was using the job search terminals at the centre.
He said he could clearly hear staff as they interviewed other clients and dealt with claims on the phone.
"The staff quite loudly broadcast this information, which I easily overheard from the other side of the room," he told BBC Southern Counties Radio.
Identity fraud expert Neil Munroe, from credit reference agency Equifax, said he was encouraged that people were more conscious of keeping personal information secure.
"Ten years ago they wouldn't have had a second thought that it could be used for criminal purposes," he said, adding that the use of soundproof booths would ensure information could not be picked up.
Ms Connolly, district manager for Surrey and Sussex JobCentres, said all managers had been asked to confirm that customers' details could not be heard or seen by anyone other than the personal adviser dealing with them.