Detectives used a covert search warrant to rifle through a murder suspect's bedroom while posing as a couple during an estate agent's open day and sneak out with a hat and comb for DNA analysis.

The secret operation was revealed on the second day of Gambian butcher Sainey Marong's High Court murder trial in Christchurch.

Marong, 33, denies picking up sex worker Renee Larissa Duckmanton from Christchurch's red light district on May 14, 2016 before strangling her to death, dumping her body on a country roadside, and setting her on fire.

His defence says the evidence is best understood through a "lens of mental imbalance".

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Today, the court heard how two detectives managed to take items from Marong's flat in the Christchurch suburb of Ilam and get them analysed for DNA.

While he was identified as a suspect in Duckmanton's murder, the property where he was living was up for sale.

On Sunday, May 22 Detective Maania Piahana and a colleague posed as prospective buyers at an open day after a covert search warrant was granted. They were "tasked with gaining an item which may be suitable for DNA comparison" without Marong being alerted, Piahana told the court.

She walked in carrying an empty black handbag, which contained disposable gloves and plastic exhibit bags, and signed the register under her first name.

They walked through the address and found documentation with Marong's name on it in a middle bedroom on the ground floor, the court heard.

Renee Duckmanton who was allegedly murdered by Sainey Marong. Photo / Supplied
Renee Duckmanton who was allegedly murdered by Sainey Marong. Photo / Supplied

With a glove on her left hand, Piahana said she took a black comb from a bedside cabinet and put it inside a sealed bag.

She then also took a black baseball cap and another comb, which she later returned.

Once they had grabbed what they came for, the officers returned to the city's central police station, the court heard.

The Crown says its case against Marong is "overwhelming", with DNA samples taken from Duckmanton, and also samples found where her body was dumped, allegedly belonging to Marong.

A sheep's tongue "bizarrely" found at the scene is alleged to have come from an animal that Marong helped butcher the day the sex worker went missing.

Police examinations of Marong's mobile phone, the Crown alleges, "deeply implicates" him in the murder.

Weeks before the alleged murder, the Crown says Marong searched for what kidnappers use to make people unconscious, chloroform, and is claimed to have clicked on an article entitled, 'How to kidnap a girl: an informative guide'.

There were also multiple searches about necrophilia, including a "man having sex with dead body".

The trial, before Justice Cameron Mander, continues tomorrow.