Mail thieves have stolen cheques meant for bill payments worth around $100,000 from at least 16 people in rural Waikato, police say.
Area commander Inspector Dana McDonald said thieves targeted mailboxes with small flags raised that signalled to posties there was mail to collect.
Once they had the cheques, the offenders amended the amount they were written out for and tried to cash them at banks.
"When you think about it, for dishonest people stealing cheques is pretty easy. All you have to do is take note of the mailboxes with flags raised and get to them before the scheduled postie's run," Mr McDonald said.
Police believed they were dealing with an isolated issue in Coromandel but the problem is now thought to be widespread.