A toddler who died after his pancreas and liver were split in two cannot have received his fatal injuries from bouncing off a bed as his stepfather says, a court has been told.

Joel Loffley has denied murdering his partner's son James Joseph Ruhe Lawrence - known as JJ - while he looked after the two-year-old at their family home in the Auckland suburb of Orakei in November last year.

During a police interview, played to the High Court at Auckland today, Loffley described how he made the bed and "fluffed'' up the blankets while JJ bounced.

"When I've gone to fluff it the last time he's jumped, fallen off the bed and landed on his face, on his chin and his legs have gone up over his head.''

Loffley said JJ was wheezing afterwards but he put that down to the effects of a recent throat infection.

He told the police that JJ had filled his nappy so he gave the child a shower before changing him and putting him to bed.

The 29-year-old said his partner Josephine Lawrence arrived home, and he told her what had happened before going out with his brother. When he came home, he checked on JJ and found that the two-year-old had brown liquid coming out of his mouth.

"And then she screamed and then I screamed, and then I started to panic and then she started to scream.''

Loffley was asked by police if he had ever hit his kids and was responsible for the death of his stepson.

"I never assaulted my kids. I've got a few from assaulting my kids' mums but never my kids, ever, ever my kids. Ever.''

But forensic pathologist Dr Fintan Garavan told the court that he performed a post mortem examination on JJ, and bouncing off the bed could not have caused the toddler's fatal injuries.

During a slide show that included graphic photographs of the dead toddler, Dr Garavan said he found a total of 32 injuries, including evidence of two historic broken arms, a burn, as well as bruises and abrasions.

But none of those injuries caused JJ's death. Dr Garavan said an internal examination revealed the toddler's pancreas and liver had been severed by being forced into the child's spinal column.

He said he had seen children with abdominal injuries who had been kicked by a horse, fallen two storeys or been involved in high-speed car crashes, and none of them had a severed pancreas and liver.

Crown prosecutor Phil Hamlin asked if bouncing off the bed could have caused the fatal injuries.

The doctor replied that it may explain an abrasion under the child's chin but not the abdominal injuries.

Under cross-examination from Loffley's lawyer Roger Chambers, Dr Garavan was asked if the injuries could have been sustained if the child collided with the base of the bed.

Dr Garavan responded: "The physics just doesn't add up. It doesn't provide an adequate explanation.''

He said if it were possible, "there would be an awful lot of children in hospital with abdominal injuries''.

Another scientist, ESR forensic toxicologist Samantha Coward also gave evidence to the court today, and said cannabis had been found in the child's blood.

She said the evidence suggested that the child had injested cannabis recently but she could not say how recently.

The trial continues.