Around 40 smartphones are snatched each week in central Auckland - some from out of their owner's hands - because they are of high value and easy to steal police say.
Crafty thieves have then contacted owners and demanded cash for their return.
In the past week police have worked on two cases where they've tracked down the thieves and charged them with theft.
Auckland city central area commander Andrew Coster said people were often unaware their phones had been stolen.
"In most cases people have their smartphones in a public place, they'll leave it in their handbag or on a table and not pay attention to it," he said.
"On some occasions, phones will be taken out of people's hands while they're in the street."
Coster said smartphones were "very attractive" to thieves because they could easily on-sell them below market price.
"The recent increase in smartphones being stolen demonstrates the massive demand for these items," Coster said.
"Technology can assist in the recovery of stolen phones, as recent apprehensions demonstrate. However, prevention is better than cure and, by taking appropriate care, people can guard against their property going missing."
- Auckland Now