A man who breached home detention less than 24 hours after he was sentenced for beating his baby son around his face will fight to keep his original sentence.

But police have applied for the sentence to be cancelled and substituted with a new one - likely prison.

The 20-year-old, who cannot be named for legal reasons, was sentenced to nine months home detention and ordered to complete 160 hours of community work after he admitted assaulting his baby and partner.

He pleaded guilty to a charge of wounding with reckless disregard after the violent attack on his 9-month-old son in early 2017.

He also admitted a charge of assaulting the baby's mother with a weapon, and six further charges of male assaults female relating to her and another woman.

Less than 24 hours after Judge Denise Clark sentenced the man, he breached the conditions of his home detention.

The Herald revealed that the day after the sentence was handed down he went to the pub to celebrate.

The man pleaded guilty to the breach when he appeared in the District Court this morning.

But he opposed an application by police to have his sentence cancelled and substituted with another penalty.

He will reappear in court next week where the application and opposition will be heard.

He will remain in custody until then.

To protect the identity and privacy of his son, the man's name cannot be published.

Court documents revealed the details of the vicious attack that left the 9-month-old hospitalised with lacerations around his mouth, welts on both sides of his head, multiple cheekbone fractures, a torn bottom lip and tongue and facial grazes.

His eyelids were so swollen that the baby could barely open them.

The attack happened when the man's then-partner was cooking breakfast.

He took the baby into the bedroom after becoming frustrated with his continual grizzling - the result of teething.

Alone in the room, he struck the baby repeatedly on his face and head and grabbed him by the chin and mouth to stifle his cries.

He then forced something solid in the infant's mouth, causing a tear between his tongue and the base of his mouth.

From the next room the baby's mother could hear her partner yelling: "f*** up, shut up you little c***".

She threatened to call the police and the man turned on her, threatening to burn her with a hot pan before pushing her towards the ground and kicking her legs, hip and shoulder.

As she lay on the ground, he then fetched the baby and demanded she feed him.

Family members arrived shortly after and sought medical help for the baby after noticing bruises on his face.

The man told them an internal door had fallen on the baby.

The baby spent a week in hospital and was then placed in the care of Oranga Tamariki.

At sentencing last week it was revealed the man had also assaulted the woman on previous occasions between September and October 2015 - including while she was pregnant.

On one occasion he hit her so hard with a game console that she could not open her eye.

Another time he went up behind her and held a knife firmly to her throat.

The baby's grandfather, the father of his now ex-partner, said the sentence of home detention was "a huge joke".

"There has been no justice for my grandson whatsoever - it's a slap on the hand. They might as well have given him a cup of tea and a big piece of cake as well," he told the Herald last week.

"We were expecting a jail term - I'm not happy at all."

He was in court today with his daughter and other family to see the baby's abuser appear.

It was the first time they had attended a hearing.

The grandfather said the process had been extremely upsetting.

"This has done a lot of damage to my family," he said.

Before the convicted child abuser breached home detention, police had been considering the case and whether they could appeal the sentence.