At least three people in Dunedin have fallen prey to attempted blackmail this week after sharing explicit images of themselves online, police say.

However, the issue could be more widespread, Senior Sergeant Anthony Bond, of Dunedin, said.

Southern district police issued a warning on social media. Snr Sgt Bond confirmed that over the past week police had become aware of three similar matters in Dunedin.

"Obviously, there are the reported cases, and there are the unreported ones as well."

In each of the cases, he was aware of, people had engaged in a conversation online believing they were dealing with a member of the opposite sex.

They had then exchanged explicit photographs of themselves before being issued a demand for "money or serial numbers for Apple vouchers", Snr Sgt Bond said.

If the payment was not made, the images would be shared with friends or family of the victim.

"I'd say it's the vulnerable or the naive that get caught up in this," he said.

On the Southern district police Facebook page, the matter was described as "a young person" who had exchanged messages with someone on Snapchat who claimed to be a teenage girl.

After chatting online, the young person was asked to send an explicit image and did so, police said.

The young person was then threatened that if they did not pay the person behind the scam, the image they had sent would be shared widely on social media.

"Fortunately this young person did the right thing and spoke to a family member regarding the blackmail attempt, and police were subsequently contacted," police said.

Snr Sgt Bond said police initially investigated to see if they could hold the party accountable — that is whether the scam originated from within New Zealand.

If the scammer was overseas, police would look to pass the matter on to another jurisdiction, he said.

Police urged parents to make themselves familiar with Netsafe.

- Hamish McLean, Otago Daily Times