A batch was printed with incorrect numbers which would invalidate the vote, Durham County Council said.
New voting packs will be delivered to affected residents in Sherburn, Barnard Castle, Willington, Hunwick, Ferryhill and Sherburn.
They should be used instead of - or, if already posted, as well as - the original papers, a spokesman said.
Only the second, correct, set of papers would be recorded, he said.
Acting returning officer Colette Longbottom said: "There are only around 265 postal ballot packs believed to be affected and anyone who has been sent one of these packs will receive a new one with a letter advising them to use the replacement.
"No other packs have been affected."
Postal voters in parts of County Durham could have to vote twice after a printing error on ballot papers.