Maija suffered fatal injuries when she was in the sole care of Duff after he and his then 18-year-old partner, Melina Puhi, argued.
Justice Downs said during sentencing in October 2018 Maija was "plainly loved" and it appeared Duff was a loving father up until the night she died.
Maija was taken to her father and left in his sole care while the baby's mother was elsewhere.
Overnight, Duff inflicted the injuries but - given his claims of not knowing how those injuries were suffered - no one knows how or why.
A hearing to appeal the sentence and conviction was held on April 20 this year before Justice Murray Gilbert in the Appeal Court and the finding has just been released online.
Duff filed his appeal two days out of time but an extension of time in which to appeal was granted.
Representing Duff, lawyer Nicholas Dutch argued the minimum non-parole period "was manifestly unjust" having regard to personal mitigating factors set out in a cultural report provided to the judge.
The cultural report suggests Duff was raised in a severely deprived urban Māori community. Duff did not succeed in the education system and failed to achieve appropriate developmental milestones in adolescence.
"Lacking the ability to control anger, respect social rules and feel empathy for others, he engaged in disruptive behaviour ultimately leading to his recruitment into the Mongrel Mob," the report said.
The report writer said the "Mongrel Mob ethos has been to invert what would be normally acceptable and turn social failure into perverse achievement".
The report said Duff tried using strategies he had learnt from a violence prevention programme he attended in prison but he experienced challenges reintegrating into society.
The report said the homes of family and friends were unsuitable for him to live in as they had gang affiliations or records of offending and he tried to find work but was restricted because of his criminal record. He said he struggled financially on the unemployment benefit so he turned to selling drugs, which eventually saw him become a methamphetamine addict.
Dutch's submission said there was a "causal nexus" between Duff's disadvantaged background and his offending.
Justice Gilbert's appeal finding said the sentencing judge took this into account in not increasing the minimum non-parole period given Duff's lengthy history of serious violent offending - which included being jailed for wounding the mother of his four other children by smashing her pelvis with a hammer - and the fact it was a "callous and brutal murder of a defenceless and highly vulnerable infant".
Justice Gilbert dismissed the appeal against conviction and sentence and declined an application to present further evidence in support of the appeal against conviction.
