The Crown is being urged to appeal against the jail terms of less than six years handed down to two men who wrongly accused a Christchurch father of being a paedophile, poured petrol over him, and burnt him alive.

Shay Barry Webster, 22, and Jason Alistor Barr, 44, pleaded guilty to the manslaughter of 55-year-old Terry Martin Smith on Anzac Day last year.

The Crown said today it didn't pursue a murder conviction on the basis that it would've been too difficult to prove murderous intent.

But Justice Cameron Mander's sentences today have been blasted by Mr Smith's family as being "totally inadequate".

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"We are concerned that both offenders pose a high risk of reoffending given that between them they have over 100 prior convictions. Many of their convictions are for violent crimes," said Mr Smith's ex-wife Michelle Smith outside court.

"We want to stress to the general public that this could be you in this situation, where a loved family member has been taken away and the value of their life has been minimalised by what we believe to be a totally inadequate sentence."

A packed public gallery heard how Webster and Barr - both of Christchurch - were of the mistaken belief that Mr Smith was a paedophile.

At a family gathering in Woolston, the drunken pair accused Mr Smith of having a sexual interest in children.

Mr Smith vehemently denied the allegations - his innocence later confirmed by police - but was cornered in his bedroom where Barr poured petrol over him.

When another man came into the room they were calling him "a dirty paedo", the court heard.

As the witness moved to diffuse the situation, Webster flicked a lighter twice, which ignited the petrol fumes and set Mr Smith on fire.

Mr Smith died in hospital two weeks later with burns to 29 per cent of his body, surrounded by his traumatised family.

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"This was not a quick death but an agonising one," Justice Mander said.

Ms Smith, Mr Smith's former wife, told the court today that she struggled to see how anyone could treat another human being that way.

Their 19-year old son Aaron Smith told of his shock at seeing his father in hospital that first night, where he "kept remembering the smell of petrol around my dad".

"I would not wish this on anyone," he said.

Crown prosecutor Mark Zarifeh said the pair embarked on vigilante justice.

Judge Cameron Mander found that both killers, who had troubled childhoods, were equally culpable.

Barr was jailed for five years and seven months with a minimum non-parole period of two years, nine months.

Webster was sentenced to five years and three months in jail, with a minimum non-parole period of two years, six months.

Sensible Sentencing Trust spokeswoman Jayne Walker described the sentences as "crazy".

She called for the Crown to appeal.

"It's essential they appeal, given the amount of previous convictions they have and the nature of the offending - somebody has died here in horrific circumstances. The family has to live with those mental images for the rest of their lives," Ms Walker said.

"Before too long the family will be facing parole hearings and the possibility of these two guys being let out. It's not right.".

Webster was sentenced to five years and three months in jail, with a minimum non-parole period of two years, six months.