Victoria Police were ordered to stop using informing lawyer Nicola Gobbo but a detective says they couldn't "unhear" what she may have told them.
In 2010, then-chief commissioner Simon Overland ordered the force stop using the criminal barrister as a source against her gangland clients.
But a royal commission into the 'Lawyer X' scandal has been told officers didn't expect Ms Gobbo to stop coming forward with information.
"In that situation we couldn't unhear anything we were told by anybody, particularly in this situation," Detective Superintendent Paul Sheridan told the inquiry on Thursday.
Counsel assisting the commission Andrew Woods asked whether police expected Ms Gobbo would simply keep providing information "off her own bat" anyway.
"Based on prior behaviour that was a concern, yes," Mr Sheridan replied.
In 2010, the now infamous lawyer took legal action against police for allegedly failing to protect her as a witness, with a confidential settlement reached out of court.
"After she's deregistered, after she's sued, after that proceeding has settled there's a document that confirms that ... when there is information provided by her, it can be disseminated and I'm suggesting to you that that's problematic given all the water under the bridge," Mr Woods put to the detective.
"I don't think it's quite that simplistic in this instance," Mr Sheridan said.
The royal commission is looking at how Ms Gobbo ended up giving police information on-and-off, officially and unofficially, between 1995 and 2012.
Chief Commissioner Graham Ashton is due to be grilled about the scandal next week, ahead of Mr Simon Overland and his predecessor Christine Nixon.
Commissioner Margaret McMurdo has ruled Ms Gobbo must also give evidence over the phone, despite her lawyer arguing the former barrister is too physically and mentally unwell.
Ms Gobbo cannot be forced to do so and Ms McMurdo doesn't expect she will front up.
But the commissioner can recommend criminal charges against the ex-barrister for failing to comply with an order.