Gambian-born Christchurch butcher Sainey Marong pictured after his arrest on May 26, 2016.

The man who murdered a Christchurch sex worker arrived in New Zealand on a fake passport and was due to be deported.

Sainey Marong, 33, was found guilty of murdering Renee Duckmanton in the High Court at Christchurch on February 23. He strangled her during a dispute after having sex in the back of his car on May 14, 2016. He dumped her body on the side of a rural road the next day and set her on fire.

Stuff can reveal Marong arrived at Auckland Airport on a flight from Hong Kong using a fake British passport on January 10, 2014. He destroyed the passport at the airport.

SUPPLIED Duckmanton, right, with her mother, Tracy.

Immigration New Zealand (INZ) has confirmed Marong, who lived in Ilam and worked as a butcher, was in the country "unlawfully" and had been issued a deportation notice.

READ MORE:

* Inside the brutal and disturbing murder of Renee Duckmanton

* Sainey Marong found guilty of murdering Renee Duckmanton

* Killer says he was 'definitely' insane when he strangled sex worker to death

* Murder-accused Sainey Marong Googled 'can fire destroy DNA'

* 'Distinctive' tail light helped police track murder-accused, court hears

* Woman followed by murder-accused Sainey Marong in weeks before sex worker's death, court hears

* Undercover cops pose as potential buyers at open home to get murder suspect's DNA

* Man on trial for murder of Christchurch woman Renee Duckmanton had 'desire to kill', Crown says

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GEORGE HEARD/STUFF Marong told the court he was "definitely" insane when he killed Duckmanton.

When asked by Crown Prosecutor Pip Currie during his two-week trial if it was OK for him to travel to New Zealand on a fake passport, he said "yes".

"So many hundreds of thousands are living in the community that arrive as Mr Marong arrived in New Zealand," said Marong, who often spoke in third person.

Marong was raised in Banjul, the capital of Gambia as part of the Mandinka tribe. He later told a forensic psychologist that he joined the United Democratic Party at the age of 16 when a university student was killed with no investigation or consequences.

The court heard that he witnessed the massacre of 14 students after a demonstration against the regime of president Yahya Jammeh​ in 2000.

He began a computer science and commerce degree at university before dropping out to become a taxi driver as he did not believe his degree would help him long-term.

In August 2012 he fled Gambia, leaving behind his wife and three children, after a friend told him the regime were after him due to his political ties. Fearing he would be detained and killed, he ran.

"My life was at stake," he told the court.

"I fled to my neighbouring country Senegal. That was the easiest border crossing I could have landed myself in."

He then went to Hong Kong for 14 months before arriving in New Zealand.

An INZ spokesman confirmed Marong was in New Zealand "unlawfully" and had been served a deportation order.

Further details of his time in New Zealand were currently suppressed.

Marong's high school friend Lamin Ceesay​, now based in China, told Stuff learning about Marong's case was "the most shocking news" he had read in his life.

Ceesay has been friends with Marong since they both went to Gambia Senior Secondary School. He said Marong would give him free rides in his taxi in Gambia.

"Knowing him as that decent, kind and brilliant person from way back in high school, would be a remote possibility to digest this.

"[I] would never believe he's capable of committing such horrendous acts, not even in my wildest dreams."

The two met again in Hong Kong where Marong worked in an African restaurant as a waiter, on a low income, Ceesay said.

"Because of his strong Islamic faith and rightful upbringing he never wanted to get into anything illegal that would earn him princely sums of money, but he held on to that job despite mockery and criticism."

Ceesay said Marong called him after moving to New Zealand to say he had a job that earned him enough to take care of his family back home while also saving and was "living a life with less worry".

'HARROWING CASE'

It took the jury only 50 minutes to find Marong guilty of murdering Duckmanton.

Detective Inspector Darryl Sweeney said it had been a "harrowing case for everyone involved" and the guilty verdict was the right result.

"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."

In court Marong claimed he was suffering from a "disease of the mind" when he killed Duckmanton.

Psychiatrist Dr Erik Monasterio​ said Marong's proposed defence was "farcical".

"In my opinion this proposition is preposterous. I have no evidence of what's being proposed here."

Crown prosecutor Pip Currie said the insanity defence was "clutching at straws".

"[This is] a man who has nothing else to put forward in way of defence. He's fabricating his evidence as to what was going on in his mind."