Three knifepoint robberies of people selling products on social media 'buy and sell' pages appear to be connected, Christchurch police say.
Police said all three robberies happened in Aranui in Christchurch within the last week.
In each case, the would-be buyer of a mobile phone instead threatened the seller with a knife, before taking the phone without paying and then running away.
Detective Mike Kneebone said all three exchanges had been arranged through social media 'buy and sell' pages and people should act with caution when arranging transactions online.
"Sales should always be completed in public, preferably with a trusted companion and in sight of CCTV cameras," Mr Kneebone said.
"Check who you are dealing with by searching for feedback and other comments relating to this seller to see what other people have experienced."
Police believed all three robberies were linked.
"We ... are following strong lines of enquiry, but we're urging those buying or selling goods through unknown people, to act with caution," Mr Kneebone said.
Anyone with similar reports or information about the incidents should contact police, he said.