A woman says she was followed by murder accused Sainey Marong just weeks before he allegedly killed sex worker Renee Duckmanton, a court heard today.

Marong, 33, denies picking up 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.

The Crown claims its case against the butcher originally from Gambia is "overwhelming", with DNA samples taken from Duckmanton, and from samples found where her body was dumped, allegedly belonging to Marong.

His defence at the High Court murder trial in Christchurch says the evidence is best understood through a "lens of mental imbalance".

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Today, a woman who has interim name suppression told the court how she left work in inner-city Christchurch at 10.30pm on April 4, 2016.

As she was driving on Moorhouse Ave, she noticed a silver Audi behind her, the court heard.

Watching in her mirrors, she said the car kept turning its lights on and off, and then pulling over to the side of the road briefly, before following her again.

She thought it was "really odd" and put it down to a drunk driver.

But as she was heading home, she realised that she was being deliberately followed.

She phoned police on *555 and reported the car. Police told her to keep driving around until officers caught up to her.

Asked by Crown prosecutor Pip Currie how she felt at the time, the witness said: "Nervous. Very nervous."

She kept driving around the Hornby area, waiting for police.

Turning into a petrol station forecourt, she did a u-turn while the Audi driver waited at the entrance for her.

She was too nervous to even look into the Audi's window, she said.

She soon saw a police car flashing its red and blue lights and pulled over.

Constable Zeb Harland told the court he pulled over Marong after 11pm.

Marong denied drinking and explained that he had simply gone for a relaxing drive around the city to unwind after work, Harland said.

He also denied following anyone, saying he had been driving around aimlessly.

The police officer said he appeared calm and relaxed. After checks, he found Marong had no criminal history and carried a full New Zealand drivers licence.

Harland issued Marong with a verbal warning about his driving and let him go.

The woman was told of the outcome and she went home.

The trial, before Justice Cameron Mander, continues with more cellphone data evidence.