Former detective Roger Rogerson has told the NSW Supreme Court he believes he is a victim and took part in the disposal of a body only because he was helping out a mate.

The 75-year-old said his decision to help his friend, Glen McNamara, had "complicated" his situation and in hindsight he wished he had called the police.

Mr Rogerson and Mr McNamara are on trial for the murder of university student Jamie Gao.

But one of the reasons he did not call the police at the time was because of a deep distrust he had for them.

"Police make mistakes, look at the Lindt cafe, some big mistakes made there," Mr Rogerson said.

"I don't trust the police, I haven't trusted the police for a long time," he said.

Mr Gao, 20, was shot dead inside a south-western Sydney storage unit on May 14, 2014.

Mr Rogerson's case is that Mr Gao was dead on the ground when he walked into unit 803.

But Glen McNamara claims it was Mr Rogerson who shot Mr Gao twice in the chest.

During cross-examination Mr McNamara's barrister, Gabriel Wendler, asked Mr Rogerson why he helped to dispose of the body.

"The only reason that you assisted in removing the body of the deceased into the white station wagon was because you were helping out a mate …?" Mr Wendler asked.

"Pretty well, that's right. It was a huge nightmare what had occurred," Mr Rogerson responded.

"Why didn't you go to the police?"

"I wish I had of."

Mr Wendler asked Mr Rogerson if he thought he was a victim.

"I've never thought along those lines but that is a very good description, I have been a victim," Mr Rogerson said.

"So you're a victim on trial?

"Yes."

The public gallery, and even some jurors, giggled every now again during Mr Rogerson's evidence but there was loud laughter when he later said "I should have stayed at home".

The subject of the cross-examination at one stage turned to the gunshot residue found on Mr Rogerson's cap, shirt and pocket pants.

Mr Rogerson argued his clothes had been "contaminated"  when he was helping to move Mr Gao's body into a silver surfboard bag.

"I assisted in handling his body, I moved his arms onto his chest, I helped put the silver board cover over his body, and I would imagine that this type of [gunshot residue] would have easily ended up on my hat – from me touching the hat with my hands," he said.

"I handled the body extensively."

Mr Wendler suggested the gunshot residue had ended up on Mr Rogerson's clothing because he was the one who shot Mr Gao.

"That's not the truth, that's not the case at all," he said.

Mr Wendler suggested Mr Rogerson had shot Mr Gao because he refused to hand over the drugs he had brought to a meeting.

"Untrue, untrue, untrue … lies and rubbish," Mr Rogerson responded.

During his evidence Mr Rogerson denied he had shot Mr Gao and then threatened the life of Glen McNamara and his daughters.

He said Mr McNamara's version of events was "lies, lies, lies".