The father of slain sex worker Renee Duckmanton is demanding to why her killer was allowed to stay in New Zealand after arriving on a fake passport and going on to commit murder.

Gambian-born butcher Sainey Marong was jailed for a minimum of 18 years at the High Court in Christchurch today in what a judge described as a premeditated, "cold-blooded", and "particularly callous and cruel" murder.

Marong was subject to a deportation order in May 2016 when he picked up Duckmanton from the city's red light district, strangled her to death, and set her body on fire.

 Gambian-born butcher Sainey Marong was jailed for a minimum of 18 years at the High Court in Christchurch today in what a judge described as a premeditated,
Gambian-born butcher Sainey Marong was jailed for a minimum of 18 years at the High Court in Christchurch today in what a judge described as a premeditated, "cold-blooded", and "particularly callous and cruel" murder. Photo / David Alexander

But Duckmanton's father, Brent McGrath, who sat through the harrowing trial earlier this year, is disgusted that Marong was able to stay in New Zealand.

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"They know that he ditched his passport in the toilet at the airport but they don't actually know what his real name is. But despite that, and knowing his status, they gave him a tax number and said, 'Go hard'," the Christchurch truck driver told the Herald.

"Everything [Marong] did was premeditated. Why didn't they send him back to where he came from? I work hard, I pay my taxes, but I don't mind because look at the beautiful country we get to live in. Why didn't they just put him on a plane? I seriously don't get it."

Marong arrived at Auckland Airport on a flight from Melbourne via Hong Kong on a fake passport on January 10, 2014.

He destroyed the passport at the airport, the court heard.

But Marong had been living and working in New Zealand legally at the time of Duckmanton's murder.

Marong was subject to a deportation order in May 2016 when he picked up Renee Duckmanton from the city's red light district, strangled her to death, and set her body on fire. Photo / Supplied
Marong was subject to a deportation order in May 2016 when he picked up Renee Duckmanton from the city's red light district, strangled her to death, and set her body on fire. Photo / Supplied

Immigration New Zealand confirmed that "up until the date of the crime [Marong] was lawfully in New Zealand".

"Subsequent to his arrest, his status has become unlawful," said Immigration New Zealand assistant general manager, Peter Devoy.

He was unable to answer other questions put to Immigration New Zealand, citing "legal and privacy reasons".

Statutory suppression orders ban publication of other details surrounding Marong's case.

Marong was born and raised in Gambia's capital city of Benjul as a member of the Mandinka tribe.

After secondary school, he studied computer science and commerce at university before dropping out to drive a taxi.

He left The Gambia on August 28, 2012, leaving behind a wife and three children.

After going to neighbouring Senegal, he went to Hong Kong for 14 months before landing in New Zealand.

Security camera footage of Renee Duckmanton walking on Peterborough St in Christchurch at about 9pm on May 14, 2016. Photo / NZ Police
Security camera footage of Renee Duckmanton walking on Peterborough St in Christchurch at about 9pm on May 14, 2016. Photo / NZ Police

McGrath says immigration controls need tightening to prevent similar tragedies from happening in the future.

"Something has to change or else you'll have another family going through what we are going through," he said.

"It's not about blame, it's about accountability. Just like he's accountable for what he did to Renee. I have to get something good out of this."

If police hadn't caught Marong quickly, McGrath fears "he would've gone on to become one of New Zealand's first major serial killers".

"I don't know if New Zealand has ever seen anything like what he's done. He's just sick," he said.

McGrath described his daughter as a fun, caring, and beautiful woman who wanted to have a baby.

"She would've been an amazing mum," he said.

"I actually wish she'd have had one before this so there was a piece of her still walking around."

McGrath says he suspected his daughter was working on the streets.

He had confronted her earlier, and she'd denied it.

"She said, 'Dad, don't be stupid'. I knew she was lying, I've been around a bit, but I left it at that," he said.

"She was so very strong that she would've done it anyway no matter what I said. It wasn't until the police came and told me that she was dead that I knew for sure that she had been [a sex worker].

"She would've done a lot of good in this world. She was just in the wrong place at the wrong time."