Four boys have been sentenced to more than four years in detention over the death of Mandurah man Tauri Litchfield last year.

The boys, aged 15 and 16, were part of a group who set upon Mr Litchfield in March last year as he was walking home along Pinjarra Road.

The 28-year-old had been involved in an altercation with the boys moments earlier after one of them tried to steal his wallet.

He ran away from the group and hit his head when he fell from a car park wall. He later died in hospital.

The four boys had been charged with murder, but last month they were found guilty of manslaughter in the Perth Children's Court.

Three were this afternoon sentenced to four and a half years in detention.

The fourth, described by the judge as the instigator of the attack, was sentenced to five and a half years.

They will each be eligible for supervised release after serving half their terms.

Boys who killed Tauri Litchfield 'will understand': mum

In a sentencing submission, Mr Litchfield's mother Kerry Biggs said her son was "a giver to his family and friends and every community he lived in", and that his death had shocked people around Australia and overseas.

Ms Biggs said her son was about to manage a business, and that he had wanted a house and kids.

She said she shared with her son a passion for life and people.

"I will take this pain to my grave. We can never have him back," she said.

However, Ms Biggs said the boys convicted over her son's death had a future and could make a difference and "a better future for your families, communities, and your children".

"When you have your own children, then you'll understand," she said

A statement from Mr Litchfield's girlfriend Lisa Emes, a Briton who moved with Mr Litchfield to Mandurah, where he was working as a sailmaker, was read out in court.

In it, she described him as the friendliest, most polite person she had met.

She said Mr Litchfield, whom she met in Port Douglas while backpacking, had made every effort to include her in his life.

When he died, she said in her statement, it "all came crashing down".

"I held his hand in hospital screaming at him to wake up," she said.

"After his death I questioned everything."

"[I] never met anyone with so much passion.

"There isn't a single day that goes by without me feeling a sense of loss."

'Chance encounter' escalated to violence

Defence lawyers for the teens told the court that the boys had not had stable family lives.

Save for one case, they said, both parents had abandoned or neglected them, or been in and out of prison.

Mostly, the boys had been cared for by grandparents - usually grandmothers - and there was a lack of male role models.

The four boys who were convicted and sentenced cannot be identified because they are juveniles.

The lawyer for one boy, identified as "D", said his client wanted to apologise because he felt remorse for what had happened.

D had tried to pick Mr Litchfield's pocket and that when Mr Litchfield reacted he was punched.

When Mr Litchfield was on the ground, D hit him and kicked him in the head.

D's lawyer described a "chance encounter" with Mr Litchfield and said that an initial conversation was benign, but that D's violence was "spontaneous and disproportionate".

The lawyer for convicted teenager "J", meanwhile, said the boy felt "terrible for the family" of Mr Litchfield.

He was not an instigator in the events, he said, "but he did follow and he did join in".

A lawyer for the defendant identified as "J" said he was "preoccupied with his remorse" and asked for a non-custodial sentence because he had an opportunity to set himself straight.

"E's" lawyer described his client as "aspirational", as far as sport was concerned.

He said his client was usually a spectator and not a participant in what went on, that a psychological report suggested he was not mature, and that E was "unlikely to offend again".

The lawyer for "T" said he would not try to convince the judge that there should not be a term of detention for his client, because the community expected it.

However, he said, T was simply "in the wrong place at the wrong time" and "ran along with the group".

He had told his mother of the events when he got home.

Evidence of remorse 'difficult to find'

Prosecutor Amanda Forrest said that while defence lawyers had repeatedly described "a chance meeting", when Mr Litchfield's path converged with that of the boys, within minutes he had suffered a serious head injury.

She said the boys had not been dissuaded by his flight, or by him holding his arms up, and that it was "difficult to find evidence of true remorse".

She said the boys had lied to police and that none had rendered assistance to Mr Litchfield or sought it elsewhere.

Ms Forrest said the victim lay for an hour before he was found, and that he was alone for that time with his injuries.

Two of the boys also had prior convictions, which she detailed, including for indecent assault, and had resorted to violence and intimidation in two other known instances.

Another of the boys had a conviction in relation to a random bashing of a man, which involved a brick being thrown at his head.

In all of these cases, the boys were in groups.

Youths emboldened when in packs: judge

Children's Court President Judge Denis Reynolds said that while Mr Litchfield had hit D across the back of the head when the boy was trying to pick his pocket, he "did not engage in any gratuitous or excessive violence".

But he was "effectively set upon by a pack or group of four people" and was kicked or stomped on when he was on the ground.

Judge Reynolds said "when young people offend in a pack or a group", the situation can escalate, as "young people can feel more bold in a group".

He said each boy appreciated that Mr Litchfield was trying to escape, their combined behaviour made him feel extremely fearful, he felt vulnerable and there was nobody he could call to for help.

The victim was left lying motionless in the car park where he had fallen, and it would have been obvious he was not pretending injury.

"People should be able to walk in public places alone," Judge Reynolds told the court.

Mr Litchfield's death, he told the defendants, was a "tragic loss of human life" caused "by the threatening and intimidating behaviour by the four of you acting as a pack".

The boys' sentences were backdated to when they were taken into custody in 2013.

Judge Reynolds said he took into account the harshness of the months the boys had spent in detention at a temporary facility at Hakea Prison.

He called on them to use their time in detention to commit themselves to rehabilitation.

He said the families would also need help to rehabilitate.