Family members of John Sharpe, who killed his wife, New Zealander Anna Kemp, and their daughter, Gracie, with a speargun, secretly helped police to uncover his crimes.
But while some co-operated with detectives in an elaborate plan to unsettle the killer, other family members, including his parents, supported him, Melbourne's Sunday Herald Sun newspaper reported.
Sharpe, 38, was sentenced on Friday to two life terms in prison, with a non-parole period of 33 years, for the murders of five-months pregnant Ms Kemp and 20-month-old Gracie in Mornington, Victoria, in March last year.
Ms Kemp's family in Dunedin raised the alarm over their disappearance and Sharpe went public, maintaining that she had run off with another man.
Some of Sharpe's family and friends refused to believe his story and, feigning support for him, questioned him to try to pick inconsistencies.
The Herald Sun understood that discrepancies emerged over Sharpe's account of his whereabouts at the time of his wife's disappearance. These convinced detectives he was the killer.
The tactic finally helped to break Sharpe on June 22 last year, when he confessed to the killings.
During their investigation, police secretly watched Sharpe retrieve a credit card from bushes near a Mornington public toilet block and dispose of evidence in a bin.
Police spoke to each member of the Sharpe family and found relatives who were willing to join a campaign to unnerve the killer.
A relative confirmed the covert co-operation.
"We saw it as our civil duty and were quite happy to do it."
The Herald Sun said there was a bitter split in Sharpe's family after his confession, with some relatives disgusted that he was still visited in jail by his mother and two sisters.
Supportive family members initially urged him to explore the possibility of pleading insanity.
But others said Sharpe was always a dark and troubled character, who was manipulative and sly.
His niece, Amanda Williams, said she was angered by his claim that Ms Kemp had run off with another man.
"Anna was highly moral in character. The story of another man was ludicrous and a huge slur on her character and morals.
"She adored her daughter and dearly wanted a second child in their family."
Ms Williams also rejected Sharpe's claim, made during his confession to police, that Ms Kemp, 41, was controlling and kept him away from his family.
She said Ms Kemp "had a unique charisma".
Gracie was a quiet toddler, who loved lollies, painting, playing with toys and visits to the park.
* John Sharpe told police he murdered his wife, Anna Kemp, with a speargun as she slept in their bed.
* He then coolly went downstairs and slept on a foldout sofa bed.
* Sharpe used the speargun to kill baby Gracie four days later, plying himself with whisky beforehand.
* The dismembered bodies were dumped in a rubbish tip.