Sainey Marong who claimed insanity when he killed Christchurch woman Renee Duckmanton loses appeal

Sainey Marong was found guilty at the High Court in Christchurch of murdering sex worker Renee Larissa Duckmanton. Photo / Pool

A convicted killer's claims he was insane when he murdered a Christchurch sex worker have been thrown out by appeal judges.

Sainey Marong was found guilty of murdering Renee Duckmanton after a High Court jury trial in February.

The 33-year-old Gambian butcher was jailed for life with a minimum non-parole period of 18 years.

He filed an appeal against his conviction in May and represented himself at a Court of Appeal hearing in Christchurch last month, while members of Duckmanton's family watched from the public gallery.

Representing himself and speaking in the third person, Marong said he was diagnosed with adjustment disorder and put on medication after his arrest.

He believes that his trial wasn't transparent and that he has been the victim of a miscarriage of justice.

Marong argued that he suffered a combination of mental and physical impairments, and that he couldn't communicate adequately with his lawyer.

After his arrest, he was diagnosed with adjustment disorder, he claimed.

Justice Forrest Miller pointed out that two witnesses in his trial were very clear that he wasn't insane.

Marong claimed they were wrong and that his "medical impairment" was caused by a lack of medical treatment.

He said he posed a serious danger towards himself due to a lack of diabetic treatment, while also claiming he suffered from kidney failure before the killing.

A psychiatrist and a psychologist, who both assessed Marong before he stood trial, concluded that he did not meet the criteria for a defence of insanity.

Mark Lillico, representing the Crown, said all of the expert evidence said Marong fell below the threshold.

Now, in a new judgment released today, the Court of Appeal rejected Marong's claims he was insane at the time and suffering from a disease of the mind.

"Mr Marong's calculated behaviour was plainly abnormal in the extreme, but the only expert evidence about his state of mind was unequivocal; he suffered from no disease of the mind, and was able to understand the nature and quality of his acts and to distinguish right from wrong," the appeal judges concluded.

During his murder trial, Marong admitted strangling Duckmanton in May 2016 after picking her up from the city's red-light district, before dumping her body on a country roadside and setting it on fire.

But he denied that he meant to kill her, and had no murderous intent, claiming he was insane at the time, suffering mental impairment and delusional, psychotic thoughts which began after voluntarily stopping his insulin medication early in 2016.

The jury, however, sided with the Crown and its "overwhelming" evidence, dismissing Marong's claims that he had been "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".