The fate of a father who is alleged to have killed his only daughter by taking her into a cold Bay of Plenty river is now before a jury.

The jury, in the High Court in Hamilton, must decide if Tewi Savage intended to kill Arnica Savage, two, by taking her into the waters of the Rangitaiki River, or if he was suffering from a disease of the mind at the time.

Both the Crown and defence made closing statements in the 34-year-old's murder trial on Wednesday.

Acting out of anger and jealously, Savage made a conscious decision to put his daughter in the river on July 1, 2018, Crown Prosecutor Richard Jenson said in closing.

"He stripped off, took his daughter into the river, held her under and let her go. He knew what he was doing and that it was morally wrong."

Savage and Arnica's mother Santana Moses were together for 15 years and had five children together. In early 2018 the relationship broke down and Santana started seeing a close friend of Savage's.

On July 1 the family met at in small village of Te Mahoe to discuss the separation.

Things became heated when Santana told Savage she intended to move to Australia with her new partner.

To cool off Savage took Arnica for a walk in the pram. He was later found naked, crying in the blackberry bushes, saying he had "f**ked up". Arnica's body was discovered upstream.​

"He was in an angry state and a dark place. One can see how he formed the intent to end it all and take Arnica with him as a way of really hurting Miss Moses."

Richard said the jury needed to consider causation, intent, and the defence of insanity.

Savage told those at the river what he'd done that night.

"I f...ed up mum. I put her in the river, I let her hands go," he'd told his mother.

"The only logical conclusion is that the defendant has picked Arnica up and carried her over the rocks and into the river."

He may not have thought "I'm going to kill her", but Savage must have known it would cause her to die, Richard said.

Even if Savage was suffering from a latent bipolar disorder, he understood the nature and quality of his actions.

But defence counsel Shane Tait told the jury that Savage's actions were not one of a man with murderous intent but of someone suffering an undiagnosed bipolar disorder.

Savage loved and cared for his only daughter Arnica.

Given Savage was suffering from bipolar, talking in riddles and found standing naked with his Bible, Shane asked how much weight could be given to what Savage said the night he was found.

A police constable described him as rambling and questioned his mental health. Close family and friends said Savage was acting out of character. He told the man seeing his ex-partner he "loved him", and gave him a hug.

"All the experts agree he was suffering from bipolar and depression and most of the civilian witnesses point to behaviour is not normal. Something was going on his head beforehand and afterwards."

Arnica wasn't suffocated or choked by the accused, she drowned, Shane said.

"You must be sure of how Arnica got into the water and that she got into the water at the hands of the accused - if you find that's the case then the defence says at the balance of probabilities he was suffering from a disease of the mind."

Stuff.