Former detective and true-crime author Glen McNamara has denied killing university student Jamie Gao and giving false evidence during his NSW Supreme Court murder trial.

Mr McNamara and another former policeman, Roger Rogerson, are charged with the murder of Mr Gao, who was shot dead inside a southern Sydney storage shed in May 2014.

They are then accused of taking the 2.78 kilograms of the drug ice that Mr Gao had brought to them.

The pair are pinning the killing on each other.

Mr Rogerson's barrister, George Thomas, began his cross-examination of Mr McNamara on Wednesday, accusing him of deliberately giving false evidence.

"I put it to you that you have given deliberately false evidence in this court concerning the events in shed 803 on the 20th of May 2014," Mr Thomas said to Mr McNamara.

"No, I have not," Mr NcNamara replied.

Mr Thomas continued, "I put it to you that Mr Rogerson was not in shed 803 when you killed Jamie Gao."

"I did not kill Jamie Gao," Mr McNamara responded.

Mr Thomas then accused Mr McNamara of "grooming" Mr Gao from the moment he met him for the purpose of obtaining drugs.

It is Mr McNamara's case that he met Mr Gao a number of times to research a book he planned to write about drugs and Asian gangs.

"I suggest to you that, after meeting Jamie Gao in January 2014, you groomed him in order to engage in a drug supply," he said.

"No," Mr McNamara replied.

Mr Thomas also suggested that Mr McNamara, a father of two, had fabricated evidence about being threatened by Mr Rogerson.

Mr McNamara's defence is that Mr Rogerson shot Mr Gao twice in the chest and that he helped dispose of the body only because he was under duress.

He said his life and the lives of his daughters were threatened and that he tried to leave a trail of evidence behind so the police would find out what had happened.

"You have fabricated deliberately, evidence given by you in this case by claiming that you were leaving trails of evidence for the police to find, haven't you?" Mr Thomas asked.

This was denied by Mr McNamara.

"Why not just give the evidence to the police instead of leaving trails?'' Mr Thomas asked.

"Because I was threatened by Rogerson,'' Mr McNamara said.

On the day of Mr Gao's murder the court heard Mr McNamara left his apartment wearing a baseball cap with a hooded jumper over the top and dark sunglasses.

Mr McNamara said it was because he was "scared" of the triads, an Asian organised crime gang Mr Gao was associated with.

"You left your apartment dressed the way you were on the 20th of May because you had a purpose I suggest to you. Your purpose was to engage in a drug supply and kill Jamie Gao," Mr Thomas put to Mr McNamara.

"No it wasn't," Mr McNamara said.

The trial continues before Justice Geoffrey Bellew.