Corrupt police stopped successful prosecutions against drug king Tony Mokbel, Victoria's former top policeman Simon Overland believes.
Operation Posse, a major investigation into Mokbel's family, was the force's best hope to bring down the high-profile figure and his so-called cartel.
"I believe all previous investigations had been compromised. They had been compromised probably by corrupt police officers in Victoria Police," Mr Overland told a royal commission into the force's use of informers on Wednesday.
"If any investigation were to succeed against him and his syndicate, it needed to be run with very tight security so it wasn't compromised at a very early stage."
The investigation did succeed and when a number of Mokbel's underlings turned on him to avoid lengthy jail terms themselves, he wound up behind bars.
Mokbel is serving a 30-year prison sentence in Victoria's maximum security Barwon Prison, where he was stabbed last year.
The operation was staffed by officers Mr Overland trusted. It had to be because of a history of leaks.
Mr Overland described Victoria Police as being the most "indiscreet" organisation he'd come across.
"It leaks like a sieve," he said.
But the inquiry has heard those senior officers were fielding intelligence on the Mokbel family from Lawyer X Nicola Gobbo - the drug lord's lawyer.
When Ms Gobbo was formally recruited for her third informer stint in September 2005, one of the first questions officers asked was to "tell me everything you know about Tony Mokbel".
"If you were in a position to say 'OK boys this is all you have to do to lock him up', what would you say?," an officer known as Sandy White asked her in recorded conversation.
Mr Overland claims he didn't know Ms Gobbo was informing on her clients and has told the inquiry she wasn't central to that investigation.
"I don't believe we did need her for Posse," he said.
But her tips were used and now Mokbel is seeking to appeal a number of convictions on the grounds Ms Gobbo was actively working against him while representing him.
Mokbel's barrister, former judge Richard Maidment QC, has also revealed plans for a petition for mercy.
At least two petitions have already been filed, including one from Faruk Orman who was last year acquitted of murder after Ms Gobbo's involvement in his conviction was determined to have caused a serious miscarriage of justice.
Mr Overland also became emotional in the witness box after being questioned about one particular gangland murder, and what could have happened if they'd arrested the killer on other charges earlier.
"We made a decision not to because we figured charges of those nature wouldn't be serious enough to put pressure on him to roll," he said.
"And in the end someone died."
Australian Associated Press