Leon Jayet-Cole died of serious head injuries on May 28 12.30pm.

"We should kill a kid more often."

Police secretly recorded the remark made by the stepfather of an autistic boy only days after the boy died, an inquest heard.

Coroner Brigitte Windley began an inquest in Christchurch on Thursday into the death of Leon Jayet-Cole, 5, who was taken to Christchurch Hospital on May 27, 2015, with serious head injuries causing cardiac arrest. He died the next day.

Supplied Drug use was common in the home of Emma Roberts (nee Jayet) and James Roberts, Leon's mother and stepfather.

His stepfather James Roberts was charged with his murder and died in June, 2016, awaiting trial.

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Detective Sergeant Chris Power, who headed the investigation into Leon's death, told the inquest police had planted a listening device in the house Roberts shared with Leon's mother Emma Roberts (nee Jayet), to whom he was married.

IAIN MCGREGOR/STUFF Coroner Bridgette Windley.is looking into the death of Leon Jayet-Cole.

On June 1, 2015, Roberts and Emma could be heard talking about the gifts and money they had received after Leon's death.

Roberts had said the "new stuff" was incredible and had added "we should kill a kid more often".

He was also recorded saying he and Emma should do something cool for the kids, such as going to Fiji.

Iain McGregor The court saw the video of a police interview with James Roberts following Leon's death.

Police believed the comment about killing a kid was tantamount to an admission "in jest" by Roberts who was probably on drugs at the time, Power said

Police were well justified in laying the murder charge against Roberts, he said.

They believed Roberts had regularly beaten Leon and his autistic brother over several years in a home environment that featured drug use by both Roberts and Emma, he said.

Iain McGregor/Stuff Emma Jayet enters court for an inquest into her son Leon's death..

Traces of methamphetamine and heroin were found in Leon's hair.

Roberts had told lies about a hand injury he had at the time of Leon's death and had explained a fresh hole in a plasterboard wall with a story about his "dog's bum" that was "unfeasible".

His story about what happened to Leon was not consistent with phone calls and the severity of injuries sustained by Leon. His accounts varied on important points.

Leon's post mortem showed a history of injuries not consistent with play, rough–housing or ordinary falls, he said.

Leon already had a broken big toe and facial grazes, sustained in the weeks before the major blow to his head leading to his death.

In his evidence, Power recited 44 abrasions and bruises to Leon's body when he was admitted to hospital on May 27, 2015.

Medical reports showed Leon's head and other injuries were more common in road accidents and of falls from 20m. Kicks, stomps, and punches could also have produced the injuries.

The inquest on Thursday heard a disturbing recording of Roberts' 111 call during which he was instructed how to give Leon mouth-to-mouth resuscitation and CPR.

Police later recovered a text Roberts sent to Emma as Leon was taken away by ambulance.

"Apparently I saved his tiny wee life," the text said.

The inquest heard Roberts phoned the emergency services at 1.34pm on May 27, 2015, to report that Leon was not breathing.

He later told police he had heard a bang around 1.25pm when he had been on the phone with Leon's mother.

However, police found Roberts had called Emma, who was in hospital being checked for pregnancy complications, shortly before 1pm, Power said.

Emma's midwife had overheard Emma referring to a bang and telling Roberts to keep Leon awake and to keep poking him if necessary.

In a sometimes tearful police interview on the day Leon was injured that was played to the inquest on Thursday, Roberts said he had picked up Leon from school just after midday and taken him home.

While he was on the phone to Leon's mother Emma, he heard a bang like something had hit the floor- but didn't hear tears.

He had continued talking to Emma and gone to check on Leon after about five to 10 minutes. He noticed Leon was very pale and his head and hand were at funny angles. He was very pale and cold to touch.

The inquest continues on Friday.