A mafia killer who allegedly organised a shooting in which a three-year-old boy, his mother, and her partner were all killed has been arrested in Italy's southern Puglia region, local media reported.

Giovanni Di Napoli, who officers took in to custody on Monday, March 16, is believed to be behind the slaying of toddler Domenico Fornari, his mother Carla, and her partner Mimmo Orlando, almost a year ago to the day.

The woman's two other children, aged six and eight, survived by playing dead, reports Ansa.

The children's biological father, a mafioso, was murdered in 2011, reports the news site, citing police sources.

Carla Fornari provided information leading to the arrest of several criminals, while Orlando had used his day out of prison to attempt to re-establish control over drug dealing networks in a region of the town, investigators said shortly after their murder,

Di Napoli, who holds convictions for murder and mafia associations, is accused of organising the murder of the family after getting into a personal dispute with Orlando.

At the time of the shooting, Fornari was driving Orlando back to prison, where he was on short term release.

After the shooting, experts said the crime demonstrated that the mafia's so-called 'code of honour' was a myth, and that the organisation targeted innocent children with impunity.

Only two months before the murder of Domenico, three year old Nicolino Campolongo was killed in a suspected mafia hit along with his grandfather and the man's partner, with their charred bodies found in a burnt out car in the Calabria region.