People selling mobile phones online have been robbed when they went to meet the buyer in the Christchurch suburb of Aranui, police say.

Each time, a knife was presented to the seller of a mobile phone before the offender fled with the phone without paying.

It has happened three times in the last week, including on Sunday.

Detective Mike Kneebone says in all three incidents the buyer arranged to meet in the Shortland Street area of Aranui at various times of the day.

"We believe all three incidents are linked and are following strong lines of inquiry, but we're urging those buying or selling goods through unknown people, to act with caution," he says.

"Sales should always be completed in public, preferably with a trusted companion and in sight of CCTV cameras."

People should search for feedback and other comments relating to the person they are dealing with, Detective Kneebone says.

"Most importantly, if you're uncomfortable with the person you're doing the sale with, don't go through with the transaction."