Underworld lawyer Nicola Gobbo was already in "serious difficulty" when she became a police informer.
But former police boss Simon Overland believes the force made the "least worst decision" in recruiting her as a gangland informer 15 years ago.
She was stuck in a job with no option to resign - representing drug kingpin Tony Mokbel and his family of crooks.
He was never going to let her leave his side, and to do so alone would have been a "death sentence", the once chief commissioner told a royal commission into Ms Gobbo's informing on Monday.
"She had become a facilitator of criminal conduct, not a legal adviser," he said.
Mr Overland knew Ms Gobbo was an informer almost immediately after she was recruited in September 2005.
He was an assistant commissioner at the time, and later deputy. He cannot remember if he told his boss, then chief commissioner Christine Nixon.
Mr Overland was surprised and concerned as Ms Gobbo was the first barrister he had heard of becoming an informer.
But he said he never became aware she was giving up privileged information or dobbing on her clients.
He says it is coincidental that weeks after she was signed up, officers started an operation to bring down her drug kingpin client Mokbel.
He rejected that it was an "innovative technique" to have a lawyer inform on her clients, despite one of the first questions handlers asked her was to tell them everything she knew about Mokbel and his crew.
Mr Overland later learned Ms Gobbo represented a gangland murderer who had rolled on others, including underworld killer Carl Williams and another man who cannot be identified.
She then went on to represent the third man.
"It was a very complicated set of circumstances," Mr Overland said.
In his statement he said it was done with the full knowledge of then-director of public prosecutions Paul Coghlan and senior crown prosecutor Geoff Horgan SC.
But in the witness box he admitted that was not true because he had not told either of them Ms Gobbo was also an informer.
"So you didn't give them the full knowledge," counsel assisting Chris Winneke QC asked.
"No," Mr Overland replied.
He described the use of human sources as "invariably legally and ethically complex" and requiring balance.
Mr Overland was also grilled about the significance of Ms Gobbo representing a killer-turned-witness when she herself had been the subject of interest from investigators.
"In the murky world that was operating then, no, it wouldn't have been at all unusual," he said.
But he would have been concerned if he had known then that Ms Gobbo was representing a witness who was rolling on Carl Williams, who was also her client.
"Well, it would have raised my concerns if she was doing that, because that probably would be a criminal offence," he said.
Mr Overland, who was last week sacked as chief executive of the embattled Whittlesea Council, is due back in the witness box on Tuesday.
Australian Associated Press