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Owners are being urged to post back voting papers for residents no longer living at that address.
The best thing people could do if they had voting papers for unknown residents was to readdress them and put them back in the mail, electoral officer Pam Jordan said.
It was possible people put unwanted papers in the rubbish bin but it was more helpful if they were marked as ''not at this address'' and sent back, she said.
That enabled the electoral commission to follow up on the residents to ensure the correct address was added in time for the next election.
''They do a good job, to ... make sure electoral rolls are up to date but we have limited control of what people do.''
Registrar of electors Dee Vickers said voting papers were addressed to a specific person so it was illegal for anyone else to open them.
''It is the risk of a postal vote.''
However, people were generally honest and many brought unclaimed papers to the city council or the electoral office, she said.
''I don't think there is a massive problem here. Most people try to do the right thing.''
There were also many checks and balances in the system to ensure anyone who tried to ''cheat the system'' was detected and those votes were discounted.
Halls of residence and rest-homes gathered together unclaimed voting papers and returned them to the electoral office so they were not left lying about.
It was simple to return unclaimed papers by crossing out the address and putting ''return to sender'' on the front, she said. No stamp was needed.