Marco Tapia, 23, died in his bed as the unintended target of a break-in shooting in Sydney's southwest on September 1.
Detective Acting Superintendent Brad Abdy said a firearm found in the family home where Mr Tapia was killed had been linked to the group, who travelled from Melbourne to Sydney on August 30.
Police allege the group, linked to the notorious Alameddine crime family, carried out criminal contracts for two separate shootings in Hurstville and Turrella while in the state.
The same firearm was used at all three shootings, Det Supt Abdy told reporters on Friday.
Four people in Melbourne were arrested on Wednesday, including a 22-year-old man in Epping, while a 23-year-old man, 26-year-old woman and 16-year-old boy were all arrested in Donnybrook.
The group appeared before Melbourne Magistrates' Court on Thursday, where extradition to NSW was applied for and granted.
"Criminals can't come interstate, commit offences, run away and hide back in their states and think that they're going to avoid detection and prosecution for their crimes," Det Supt Abdy said.
"It's equally as important that people who are overseas tasking criminals in Australia to commit these types of offences can't use our borders as a means of protection from prosecution and identification."
The shooting in Hurstville unfolded about 2.30am on August 31 and involved three unknown men forcing entry into a home, before a gun was fired into a wall.
Police said there was no one home at the time, and the men left the scene shortly after in a black sedan.
About 30 minutes later, shots were fired into a grey sedan parked on the road in Turrella.
"That matter's being investigated by the homicide squad. We will work with them and our evidence sharing to establish the link," Det Supt Abdy said.
"These people are obviously significant persons of interest."
Detectives quickly identified four people who had allegedly travelled from Victoria to carry out the criminal contracts.
Search warrants were executed at short-stay accommodation in Lurnea and Campbelltown on September 9, with police believing the group had stayed at the locations between August 30 and September 1.
Officers will travel with all four to Albury Police Station, where they're expected to be charged.
The four have not been arrested or charged in relation to Mr Tapia's murder.
Two boys, 14 and 16, were arrested and charged on Wednesday over their alleged roles in the murder and will remain behind bars until they next front a children's court on November 13.
The boys are not alleged to have entered Mr Tapia's family home in Glen Alpine. For that murder, three suspected offenders remain at large.
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