WARNING: Contains court evidence that some people may find upsetting.

A young father has spoken of the panic-stricken moments when his baby son was found "lifeless" in his cot - and has denied causing bruising and an adult bite mark later discovered on the infant.

Shane Neil and his partner Lacey Te Whetu had been smoking synthetic cannabis on the day their son Isaiah Neil died.

Neil said he found Isaiah in his cot, "saturated" with sweat, and woke Te Whetu up.

"He was gone. Lacey was trembling, screaming, out of control," said Neil.

"She was holding him up under the arms, saying 'my baby', those sort of words.

"I was in shock, I didn't really say anything."

Neil was giving evidence in the trial of Donna Catherine Parangi - Te Whetu's mother - on a charge of manslaughter of her grandson.

Donna Catherine Parangi is facing a manslaughter charge over the death of eight-month-old Isaiah Neil. (Photo / File)

Under cross-examination by her defence lawyer Susan Gray, Neil also conceded to having memory lapses about what happened the day his son died.

She described some of his evidence as a "figment of your imagination".

Isaiah Neil was 8-months-old when he died at his grandparents' home in Ruatoki, near Whakatāne, in November 2015.

The infant died from heatstroke after being left in a hot car for several hours, according to the Crown which alleges Parangi is responsible for his death.

She was smoking synthetics with her daughter Lacey Te Whetu, Isaiah's mother, while the infant was sleeping in the car.

Parangi pleaded not guilty to the manslaughter charge.

Gray told the jury said there was "considerable doubt" that heatstroke, also known as hyperthermia, was the cause of death.

Shane Neil and Lacey Te Whetu, Isaiah's mother, both pleaded guilty to the manslaughter of their son.

Under questioning from Crown prosecutor Richard Marchant, Neil admitted he had been smoking synthetic cannabis on the day Isaiah died.

He fell asleep but when he woke up, Neil said he found Isaiah inside the car parked outside.

His son was strapped in his carseat, unresponsive.

"[He was] sweaty, wet, hot," said Neil. "Limp, he felt limp."

Neil said the car doors and windows were closed, although he was not "100 per cent" sure the sunroof was also shut. The car was "very hot".

He took Isaiah inside and tried to check for a pulse.

"I couldn't tell, I was still pretty high. He did appear to be lifeless at the time, that's the only way I can describe it."

Neil woke Te Whetu to check on their son. "She said he was fine, still breathing, so put him back to bed."

Neil and Te Whetu both went back to sleep.

Around 6pm, Neil said he woke up and checked on Isaiah again.

His son was "saturated" with sweat, said Neil, and seemed lifeless so he woke up Te Whetu.

They stripped the clothes off Isaiah and started sprinkling cold water on him, in an attempt to cool him down.

By this point, Neil said he and his partner were in "full panic mode".

"He was gone. Lacey was trembling, screaming, out of control," said Neil. "She was holding him up under the arms, saying 'my baby', those sort of words.

"I was in shock, I didn't really say anything."

Neil said he attempted CPR, using his fingers on his son's chest, and became aware of an ambulance arriving.

Paramedics kept trying to resuscitate Isaiah inside the ambulance, while Neil waited outside, but he was pronounced dead.

"I remember saying 'we failed'….as parents," said Neil.

"I got stoned after that."

He later spoke with Detective Sergeant Max Holder and told the police officer "we cooked the baby".

Asked by Marchant why he said those words, Neil told the jury: "Because we left him in the car."

The pathologist who examined Isaiah's body after his death found extensive bruising and an adult bite mark, Marchant told Neil.

Neil denied inflicting the injuries, and did not know who had, but confirmed pleading guilty to manslaughter for his role in the death of Isaiah.

Under cross-examination by Gray, Neil was asked about five different statements he gave to police. The most recent was in May 2018.

"Across those five statements, you've changed you've account of what happened that night. Do you accept that?" asked Gray.

Neil: "That's possible".

Gray: "Because your memory of that day isn't very good, is it?"

Neil agreed he was "very stoned" on the day Isaiah died.

So stoned, Neil agreed, he went back to sleep after Isaiah was pronounced dead and the police struggled to rouse him.

He agreed being "stoned" made it difficult to remember events.

When speaking to Detective Sergeant Max Holder about Isaiah's death, Neil said he was struggling to remember what happened the previous day.

If his memory was poor at the time, Gray suggested his recollection of events was not going to improve several years later.

"I was in shock at the time," said Neil.

"You've got massive memory blanks on that day," said Gray. "How on earth can you say you removed Isaiah from the car at 3.30pm?"

Neil: "It was an estimate."

Gray suggested Neil removed Isaiah from the car before his two older children returned from kōhanga at 2.45pm, which he disputed.

He could not remember his children being dropped off at all.

In his first statement to police on the day Isaiah died, Neil said he didn't know what time he took his son from the car.

He agreed with Gray his estimate of 3.30pm was a "guess".

Neil could not recall how he first saw Isaiah in the car, as he thought the windows were tinted.

Parangi told police she left the doors open to ventilate the car, said Gray.

What is possible Neil saw Isaiah because a door was open?

He maintained the doors were closed, although still uncertain about the sunroof.

Gray pointed out several memory lapses by Neil.

"How on earth can you remember the doors being closed?"

Neil said the doors were "one of the things" he did remember.

Parangi's defence lawyer also highlighted Neil never mentioned to police checking Isaiah for signs of life, after removing him from the car, until his fifth statement to police in May 2018.

This was despite giving a two-hour interview on video during the police investigation before Neil was charged with manslaughter.

Under cross-examination by Gray, Neil said he went back to sleep after trying to revive Isaiah with Te Whetu.

Gray said Neil's timeline of events, including picking Isaiah up from the cot, was a figment of his imagination. She suggested Neil was asleep the whole time.

"That's incorrect," said Neil. "I remember waking up and checking on him."

The trial in front of Justice Sally Fitzgerald in the High Court at Hamilton is expected to last up to three weeks.