The woman convicted of killing two year old Moko Rangitoheriri is arguing her 17-year jail sentence is too tough, given her mental state at the time.

Tania Shailer and her boyfriend David Haerewa were each jailed for 17 years in June, after pleading guilty to the manslaughter of Moko who had been left in their care.

An appeal by the pair has begun at the Court of Appeal in Wellington.

Shailer's lawyer Ron Mansfield says the court failed to acknowledge that Shailer's poor mental health lessened her culpability.

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Mr Mansfield says for that reason, Shailer's sentence should be more lenient.

He told the court, if the offending is driven by an offender's poor and unmanaged mental health - rather than a desire by them to be deliberately cruel - then that has to be understood and properly recognised.

However the Crown contended that argument was misleading.

Lawyer Brendan Horsley told the court there was clear intent in Shailer's actions.

He says she talked about wanting to punish Moko for him hurting one of her children.

"She's not talking about some sort of dissociative state.

"She has a clear intention of causing harm from there. She bites that child."

- Newstalk ZB