People who have put mail in a Whanganui postbox over the past few weeks should check it reached its destination.
Police also want them to check their bank accounts or credit cards for "strange transactions" after catching a man who allegedly fished mail out of postboxes.
Police were investigating the theft of the mail, Sergeant Nick Brunger said.
They had identified several victims and the thefts were predominantly in Aramoho, but also in a couple of postboxes in town.
"The perpetrator has whipped up a device with the old hook-and-line type of system, and he's indiscriminately hooking on to the mail and pulling it out," Mr Brunger said.