"Someone has stolen the cheque from her mailbox, deleted the amount and payee and turned it into $10,067," he said.
Three other cheques also stolen recently from a private rural mail box on Tram Rd have also been cashed fraudulently.
Detective Sergeant Barnett said police were contacted last Thursday by a Tram Rd resident who said he'd posted four cheques in the rural mail system recently "and three have come back as being fraudulently altered".
"Clearly there have been systematic thefts from the mail system in the Swannanoa and Eyreton area," he said. "It's a worrying trend."
Police were checking bank security camera images to try to identify people who had cashed the stolen cheques.
Detective Sergeant Barnett urged rural residents who regularly use the mail system to check their bank balances.
He said residents should also be vigilant, take note of any people who "don't fit in" and record the registration number of any suspicious vehicles in the area.
Five people have been arrested and charged in relation to the Selwyn mail thefts, including a middle-aged Springston woman and her 17-year-old son.
Police said the offenders allegedly used details from mail stolen from letterboxes across the district to phone homeowners and convince them to hand over personal details that gave the thieves access to victims' bank accounts.
More than $41,000 is alleged to have been stolen from a number of bank accounts.