Brendon Gallo, 21, said he was "scared for his life" when he followed instructions, helping stuff Mr McNeill's body into a suitcase before setting fire to it in a reserve in Picnic Point in July 2014.

Brothers Haysem and Azam Charbaji are on trial in the NSW Supreme Court over the murder of Mr McNeill, accused of beating, stabbing and force-feeding him drugs in a weekend long ordeal.

Haysem's then girlfriend Lexy May Jamieson is also charged with being an accessory after the fact of murder, after she allegedly sought advice on how to dispose of a body and helped clean up the crime scene. The trio have pleaded not guilty.

The Crown's case is that the brothers murdered Mr McNeill following a dispute over a $3000 debt involving his on-off girlfriend Jade Patterson.

Mr Gallo needed to take two breaks from giving evidence on Monday as he was overcome with emotion while describing what he had seen and heard in the hours leading up to and after Mr McNeill's murder.

The court heard that the brothers repeatedly punched Mr McNeill in the face when he arrived at Haysem's apartment in Heckenberg in Sydney's south west, and asked for money to help repay a debt he owed.

Mr Gallo said Mr McNeill screamed as one of the brothers stabbed him in the lower back with a steak knife. Chux wipes were then taped to Mr McNeill's stab wound and he was "fed Xanax and tied up and put in the laundry".

Scared that the brothers would turn on him, Mr Gallo said he guarded the apartment's laundry door overnight and the next morning helped them transport an injured Mr McNeill to Azam's house in Cartwright.

Mr Gallo said the brothers took Mr McNeill into the bathroom and about thirty seconds later he heard a scream and he was asked for "some Chux and a plastic bag".

Asked by Crown prosecutor Margaret Cunneen why he did not leave, Mr Gallo said "I was too scared to move" and that he was afraid that the brothers would hurt him.

Mr Gallo said he was told to get more Xanax pills because Mr McNeill was screaming.

When he returned to the house to drop the pills off, he said he saw Azam with a ratchet from a toolbox and heard "three bangs" and "a lot of screaming".

Mr Gallo said he was told to purchase a large suitcase and after going to Kmart returned to the Cartwright apartment where he had last seen Mr McNeill.

"As I walked inside there was this smell and then there was Azam with Dane in the back room in sheets and another suitcase," he said.

"Azam asked me to help him move it from the old suitcase to the new one."

Mr Gallo said he helped load Mr McNeill's body in the bigger suitcase and dragged it through a reserve in Picnic Point before he doused it in petrol and set it alight.

He said he later "stared at my roof for hours thinking it was all a dream" and heard about a garbage man discovering Mr McNeill's body on the news.

Mr Gallo is due to be cross-examined on Tuesday.