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.

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"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.

"What we're just wanting is a call-out to members of the public, especially in the Aramoho area, to check on mail they've sent in the last two to three weeks. We're trying to determine or identify people that have had mail taken."

Mr Brunger said credit card or debit details found in the mail had been used to commit fraud.

"Some people have been posting direct debits that disclose their bank details and credit card information."

Mr Brunger said he believed multiple people had fallen victim to the crime.

-Anyone who believes they may be a victim or who has information can call Constable Jonathan Robertson-O'Byrne at Whanganui Police Station on 06349 0600.