Gangs are using women as young as 16 to target unsuspecting men who are then being beaten, kidnapped and robbed.
The tactic is called "honey-trapping."
Police say women are reaching out to men online who arrange to meet them. Gang members and associates then get involved.
In late December, a 45-year-old was beaten and robbed of his car, cellphone and wallet after he picked up two 17-year-olds from a Woolston address. He had met one of them online.
After driving for a short time, the teenagers asked him to pull over so they could have a cigarette.
Detective Sergeant Daniel Isherwood said two men associated with the Tribesmen gang pulled up in a vehicle.
The 45-year-old was then allegedly kicked in the head through the window, while the other man climbed into the passenger seat and punched him in the head.
He was then forced into the backseat of his car and told to sit on his hands while being driven around the city. He was taken back to his own house, where the alleged offenders took a Holden Commodore.
He was also told he owed them $5000. He did not pay the money and went to the police.
Two brothers have been charged with aggravated robbery and kidnapping.
Police are also investigating whether there is an online link to a brutal attack on a 59-year-old just over three weeks ago.
He was punched, kicked and stomped at his Rolleston home by gang members and associates.
His cellphone, wallet, four firearms, a Ford Falcon, a Harley Davidson motorcycle and a laptop were stolen.
Police arrested Black Power gang member William Hewinson in relation to the incident, who has since pleaded guilty in the district court to charges of aggravated robbery, kidnapping, burglary and unlawful possession of firearms.
Two men and a 21-year-old woman have been charged with the same offences.
Detective Sergeant Isherwood emphasised the gang tactics were not widespread and victims were only a "small part" of the community.
"They look for any opportunity really, any weakness . . . these victims are seen as an easy target," he said.
Detective Sergeant Isherwood is also concerned that young people associated with gangs may not realise how serious the penalties are for aggravated robbery.
"If a senior gang member is asking you to partake in an activity which involved threats, extortion and the harming of people, the likely outcome for you is time in prison," he said.
"The convictions for aggravated robbery and/or kidnapping will likely result in lengthy terms of imprisonment, both of which carry maximum terms of imprisonment of 14 years.
"That's not an outcome anyone wants to see for these young guys. We are open to help those who are interest in employment options, other than getting involved with the gangs. Police work with a number of external agencies who assist with this.
"These recent arrests should send a clear message to those involved in gangs that they should think very carefully before carrying out this type of offending," said Detective Sergeant Isherwood.
He is calling for victims of this behaviour to come forward. "Police are willing to support any victims of this offending and encourage anyone to come forward and speak with us. We have a good resolution rate for those who speak with us about this."