Upper Hutt man William Wakefield has admitted killing his 5-month-old stepson, but denies it was murder.

Wakefield appeared in the High Court at Wellington this morning where he pleaded guilty to manslaughter over the June 11 killing. He entered a not guilty plea to a charge of murder, and another charge of wounding with intent to cause grievous bodily harm.

The wounding charge relates to dates between early May and early June, before baby Lincoln Wakefield died.

Emergency services found Lincoln seriously injured at an Upper Hutt home last year.

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The baby was treated, including undergoing surgery, but died in hospital shortly after.

Wakefield, 32, was in a relationship with the baby's mother, and would care for the child while the mother was at work.

He remains in custody and will reappear in court in March.