A seemingly calm Sainey Marong filled up with petrol with the body of murdered sex worker Renee Duckmanton in the back of his car just before he dumped her and set her on fire, newly released CCTV footage shows.

Marong, a 33-year-old butcher originally from The Gambia, was found guilty of murder by a unanimous jury after a two-week trial at the High Court in Christchurch.

He had admitted strangling her to death after picking her up from Christchurch's red light district but denied that he meant to kill her, claiming he was insane at the time.

Now, the security camera images, along with others that show 22-year-old Duckmanton's last movements before she was killed in May 2016, can be shown for the first time.

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Sainey Marong in the dock of the High Court in Christchurch. Photo / File
Sainey Marong in the dock of the High Court in Christchurch. Photo / File

They show Marong in his silver Audi cruising Manchester St on the evening of May 14, 2016, before picking Duckmanton up.

Just after 9pm he's seen turning into Peterborough St and, seconds later, Duckmanton can be seen walking past the private security camera of a resident of St Luke's apartments and out of sight.

At 9.36pm, Marong is captured on CCTV again, withdrawing cash from an ATM on Riccarton Rd.

CCTV, bank transactions and cellphone records – along with DNA – were central to police building an overwhelming case against Marong.

During his evidence, Marong said that after he had sex with Duckmanton in the back seat of his car after pulling over somewhere in the Templeton area, she then wanted to return to Manchester St.

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

Sainey Marong, found guilty of murdering Christchurch sex worker Renee Duckmanton. Photo / Supplied
Sainey Marong, found guilty of murdering Christchurch sex worker Renee Duckmanton. Photo / Supplied

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

The next day, he's seen at 6.46pm driving into the forecourt of the Mobile service station in Rakaia, off State Highway One.

He fills up and goes into to pay, giving the attendant a thumbs-up, hesitating, and then goes back to apparently pay some more money before leaving.

Duckmanton's burning body was found by passing motorists around 45 minutes later on nearby Main Rakaia Rd, just off SH1.

Marong's DNA matched samples taken from Duckmanton, and was also directly linked to samples found where her body was dumped.

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

Sainey Marong, found guilty of murdering Christchurch sex worker Renee Duckmanton. Photo / Supplied
Sainey Marong, found guilty of murdering Christchurch sex worker Renee Duckmanton. Photo / Supplied

The jury dismissed 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".

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.

Forensic clinical psychologist Ghazi Metoui found that Marong reported a settled mental state over the period "leading up to, during and subsequent" to the killing.

Dr Erik Monasterio, a forensic psychiatrist, found Marong's claims of insanity to be "farcical" and "preposterous".

After the guilty verdict, Detective Inspector Darryl Sweeney said it had been a "harrowing case for everyone involved".

"The result won't provide closure or solace for Renee's family, but it is important for everyone that they have answers and that the offender has been held to account," he said.