The man, known as Mr Chan, says he got a phone call from Joon Seong Tan on February 2, 2021 asking him to come to his girlfriend's home in Epping.
When he arrived, Tan told him he had argued with Ms Zhang because she brought another man to her home.
"He said ... he was so angry he lost his mind and killed his girlfriend," Mr Chan told the Victorian Supreme Court on Thursday where Tan is standing trial for murder.
Through a Cantonese interpreter he said Tan told him he put Ms Zhang's body in a laundry cabinet, and asked him if he wanted to look.
He refused.
"I said I'm very scared and I'm not going to have a look at the body," he said.
"I was very scared that he may also kill me so I just sat there, listened quietly."
Tan allegedly said the body was getting stinky, and asked him to go in the street and look for a rubbish bin.
He allegedly told Mr Chan he wanted to put Ms Zhang's body in the bin and get rid of it after her son had gone to sleep.
"I think now is a good chance for me to run away so I will pretend to say yes to his request, pretend to go out look for a bin," he said.
He left the house and ran for 10 to 15 minutes before catching a train into the city.
He spent time at the casino to avoid going home to the house he shared with Tan until he knew other roommates would be there.
During that time he had 50 or 60 missed calls from Tan, he said.
Mr Chan later met Tan who he said told him if the police asked, to tell him he had gone to Epping to collect $300 that was owed to him.
He did say that in his first statement to police, but explained to jurors that he had been frightened because Tan was not in custody then.
"I was not able to tell the truth because I wanted to protect my own life," he said.
He said he told the whole truth in a second statement to police, after Tan's arrest.
It's alleged Tan murdered Ms Zhang in her home sometime between 5pm and 6pm on February 2, 2021 after sharing dinner with her and her eight-year-old son.
He had allegedly previously told a friend he believed Ms Zhang was cheating on him and if he found out that was the case he would "kill her, hug her and die together".
Tan is accused of putting Ms Zhang's body in a bin and driving it to Heidelberg Heights, leaving it with others to be collected.
He allegedly later searched online for phrases including "how many years for killing a person in Australia" and "how is stinky garbage disposed in Australia".
Ms Zhang's body was discovered at a Wollert landfill site in June 2021.
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