Police investigating the 2009 Lin family murders decided soon after the crime that Robert Xie was the killer and built their case around him despite the allegations being "completely and utterly inconsistent with the evidence", a court has heard.

In his final address to Mr Xie's marathon trial over the murder of five members of the Lin family in their North Epping home, the accused's barrister, Graham Turnbull SC, said the basic facts from the crime scene suggested that his client could not have been the killer.

The Crown alleges that the 50-year-old - the brother-in-law of murder victim Min "Norman" Lin - had crept into the house in the early hours of July 18, 2009, and murdered five members of the Lin family with a hammer-like object as they lay in their beds.

But Mr Turnbull said it was impossible for one person to have committed the murders alone.

"One person did this?" Mr Turnbull asked rhetorically.

"In the dark? One person in the dark and it's Robert Xie?"

"[Is he] a martial arts ninja?"

Mr Turnbull said that, like everyone else in the community, Mr Xie was entitled to the presumption of innocence.

Yet investigating police had made him the target of their investigation within a month of the victim's funerals.

"There was a decision taken at a very early stage [about Mr Xie]," he said.

"In September 2009 police obtained medical records regarding Robert Xie. The funerals [for the Lin family] were in August. In September they were trying to determine whether he had lied about going to see a doctor or not."

Police had been unable to find strong incriminating evidence against their chief suspect, Mr Turnbull said, and had instead built their case around Witness A - a fellow inmate of Mr Xie who had given evidence against him in return for a reduction in his sentence and other inducements.

He said Witness A's evidence had been part of a year-long investigation designed to trap Mr Xie in which the informant was engaged in a "two-way" negotiation with police.

"This is a case which starts with Witness A and works backwards," he said.

"Let's face it - why on earth would the constabulary be using Witness A to prove their case [if they didn't have to]?"

He said only a fraction of Witness A's evidence had come in the form of recorded conversations with the accused killer, with the remainder being his version of what Mr Xie told him.

He said the informant had conveniently taken every piece of evidence suggesting Mr Xie's innocence and "flicked it to the boundary".

Earlier, Crown Prosecutor Mark Tedeschi, QC, concluded his final address to the jury by listing a number points from the Crown case which he said represented "powerful evidence" against the accused.

These included that Mr Xie allegedly told Witness A that he had disposed of the murder weapon the day after the murders while going to pick up his mother and father-in-law, that he had told Witness A that his wife Kathy Lin had been sedated on the night of the murders, and that a small blood-like stain on the floor of Mr Xie's garage allegedly contained DNA from four of the victims.

"Ladies and gentlemen, we submit to you that in this trial the Crown has proven beyond reasonable doubt that the accused murdered the five members of the Lin family," he said.

The trial continues.