Police said in a statement on Monday that the alleged victim, a 61-year-old man, had agreed to a meeting in Hawker on Thursday that was arranged online.

When the man arrived, police allege he was approached by two people who then threatened him with a knife.

He was forced to withdraw $100 from an ATM and items were stolen from his car, according to police.

An 18-year-old man was arrested on Saturday morning at a Narrabundah hotel and a 16-year-old was arrested later that day in Hawker.

Both will appear before court on Monday charged with blackmail and robbery.

Detective Inspector Matt Reynolds said the alleged offenders also planned to extort more money from the man at a later date.

He said it was likely the alleged offenders used a fake profile on the app as part of a "well thought out" plan, but investigations were ongoing.

Police have contacted the app developer which is assisting with enquiries.

"We're asking people to be careful when they organise to meet people online," Detective Inspector Reynolds said.

"This incident resulted from an online dating site however there are many other sites where people meet people.

"What we ask people is to ensure their own safety, perhaps take a friend with you. Do some due diligence about who you are meeting and be careful about where you are going to because you don't know these people online."

Both alleged offenders will face ACT courts on Monday on the serious charges.

"Aggravated robbery and blackmail are serious offences in the ACT, for that amount of money, no it's not worth it," he said.

"What's more important is that those people who seek to use those online dating apps to take advantage of people, we are aware of how to investigate that and in this case we've had a really good result."

Detective Inspector Reynolds said blackmail was uncommon in the ACT but police took it seriously.

"Intimidating someone for property and cash is disgraceful and unacceptable, and when this matter was reported to police, it received a swift response," he said.

"Sadly, with more people using online dating sites and online applications, some people have decided to use those services to manipulate, threaten or rob others.

"As with any online service, people need to remember they may be dealing with someone using the internet to take advantage of others.

"When arranging to meet someone through one of these sites, people should act carefully to make sure that first meeting is in a safe place."