Criminal barrister turned police informer Nicola Gobbo was representing a drug trafficker who initially offered to turn on a co-accused police officer she'd had a sexual relationship with.
It's been revealed in a royal commission Ms Gobbo was in an intimate relationship with Stephen Campbell in the 1990s but remained friends with him in 2003 when the detective and three other officers were charged over a $100,000 drug trafficking plot.
While they were acquitted at trial, Ms Gobbo's client entered an early guilty plea after initially offering to assist police in the Ethical Standards Division investigation of the other men.
The client later backed out of the deal.
The investigating officer, now retired detective senior sergeant George Tapai, said he couldn't be sure if Ms Gobbo had any influence over that decision.
"Could you be certain in a case where a person had a friendship or a sexual relationship with a co-accused, that lawyer was acting in the best interest of the client?" counsel assisting the commission, Megan Tittensor, asked.
"I guess not, no," Mr Tapai replied.
The commission's third round of public hearings is set to continue on Friday with former detective Murray Gregor, who believes murdered informers Terence and Christine Hodson would still be alive if they'd followed the advice of police.
Homicide detectives courted Ms Gobbo as a potential informer after the Hodsons were murdered in May 2004, and she was registered in September 2005.
Mr Gregor revealed at an inquest into the execution-style slaying of the couple that police warned them not to use the back room of their Kew home "under any circumstances" and particularly when dark.
Their bodies were found on the floor of that room, where they liked to watch TV at night, by one of their adult children.
© AAP 2019