The identity of a "secret source", who allegedly led police investigating the 2009 Lin family deaths to focus on Robert Xie as the main suspect is set to remain a mystery, after a judge ruled that the source must remain confidential.
Robert Xie, 48, was arrested in May last year over the deaths. Police alleged that he strangled and beat to death his brother-in-law Min "Norman" Lin and four members of Mr Lin's family in their North Epping home.
But last month, during legal argument in Downing Centre Local Court before Mr Xie's committal hearing, his lawyers said there was evidence to suggest he had been the victim of a "malicious complaint" that had "biased" the police investigation.
The lawyer for Mr Xie, Lester Fernandez, told the court that, four months after the deaths, when police were still looking for suspects, a secret source told detectives there was "conflict" and "tension" between Mr Xie and his father-in-law and mother-in-law.
This had led detectives to "craft a case" around Mr Xie and to "turn away from other avenues of inquiry", Mr Fernandez said.
Mr Fernandez applied to the court for an order that would force NSW Police to reveal the identity of the source, and any information he or she had given to them.
But NSW Police strenuously opposed the application, arguing it was a "fishing trip".
They also argued that it was in the public interest for the source to remain secret because to reveal his or her identity would discourage others from coming forward to police in future.
In the Downing Centre Local Court this afternoon, magistrate John Andrews found that, while the identity of the source was potentially important to the defence case, this was outweighed by the broader need to protect confidential sources.
"I'm of the view that, although it is likely to be material evidence for the defence, the damage done to the public interest outweighs their need," Mr Andrews said.
After Mr Andrews handed down his decision, Mr Fernandez told the court Mr Xie's lawyers would be seeking to have him discharged at the upcoming committal hearing.
"This committal will be a contested committal," Mr Fernandez said.
"All witnesses will be witnesses of substance."
The hearing will commence next Monday.