"Many of their convictions are for violent crimes.
"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."
Webster and Barr - both from Christchurch - mistakenly believed Mr Smith was a paedophile.
At a family gathering, 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 a witness moved to defuse the situation, Webster flicked a lighter twice, which ignited the petrol fumes and set Mr Smith on fire.
He died in hospital two weeks later with burns to 29 per cent of his body, surrounded by his traumatised family.
"This was not a quick death but an agonising one," Justice Mander said.
Ms Smith told the court 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 Mander found that both killers were equally culpable.
Barr was jailed for five years and seven months with a minimum non-parole period of two years and nine months.
Webster was sentenced to five years and three months in jail, with a minimum non-parole period of two years and 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," she said. "The family has to live with those mental images for the rest of their lives.
"Before too long the family will be facing parole hearings and the possibility of these two guys being let out. It's not right."