Medical experts concluded Sainey Marong did not have a defence of insanity before standing trial for the alleged murder of Renee Duckmanton.

During a dramatic day in the High Court at Christchurch, 33-year-old Gambian-born butcher Marong admitted strangling the Christchurch sex worker in May 2016 and setting her body on fire.

But he claimed he was insane at the time, suffering mental impairment and delusional, psychotic thoughts which began after voluntarily stopping his insulin medication early in 2016.

Marong spent the day on the witness stand to repeatedly explain that he was "disconnected from reality" at the time he killed Duckmanton, and that from May 6-17, he was poisoned by kidney failure that resulted in urine "travelling in my brain".

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Cruising past prostitutes and paying them for sex were the only ways a "hypersexual" Marong could calm his delusional thoughts, the jury heard.

Crown prosecutor Pip Currie asked Marong under cross-examination if he was telling the court he was insane at the time.

"Definitely," he replied.

But Currie said forensic clinical psychiatrists who completed reports on Marong concluded that he did not meet the criteria for a defence of insanity.

Medical experts found Marong to exaggerate symptoms, become histrionic, and display significant narcissistic traits, the court heard.

One psychiatrist concluded that Marong had a "considerable sense of entitlement and grandiosity", along with a high propensity to deception and manipulation.

Marong denied telling one that he found sex workers to be "dirty, immoral and inferior".

He also admitted under cross-examination lying to psychiatrists but claimed he was deluded at the time.

Marong admitted picking up 22-year-old Duckmanton from Christchurch's red light district on May 14, 2016 and engaging her services for sex.

Somewhere around Templeton, just south of Christchurch, they stopped on the roadside and had sex in the backseat of his car, Marong said.

Afterwards, Duckmanton wanted to return to Manchester St, saying the "job is done".

But Marong wanted to continue to Rolleston where he was fixated on renting a property, and he says there was an argument.

"She started yelling at me," Marong said.

He said "that voice, that screaming, that yelling was agitating me" and the dispute "triggered something very serious".

"The only method I could use to stop it was to compress her neck," he said.

Asked by defence counsel Jonathan Krebs if he meant to kill her, Marong replied, "I have no motive to cause any harm, I just did it to silence her."

Duckmanton's family in the public gallery were upset by Marong's description of events. Marong himself used a tissue to wipe his eyes.

Marong says he didn't realise that Duckmanton was dead until the next morning. He says he was so "disrupted" that he never paid her any attention because "her mouth and eyes were open" which reminded him of "the animals that normally get slaughtered at my workplace".

The Crown claims its case against the butcher originally from Gambia is "overwhelming".

It claims DNA samples taken from Duckmanton, and from samples found where her body was dumped, allegedly belong to Marong.

Marong, whose internet history in the days before the killing showed searches for necrophilia, denied having sex with her dead body.

The trial, before Justice Cameron Mander, continues tomorrow.