Victoria’s top policeman knew gangland lawyer Nicola Gobbo was a police informer before he joined the force, but said he did not know the extent of her duplicity until years later.

Chief commissioner Graham Ashton was giving evidence on Monday at the royal commission into Victoria police’s use of informants.

“I first became aware she was an informer in 2007,” he said, rejecting previous evidence that he had been told by then chief commissioner, Simon Overland, in April 2006.

Gobbo’s longest registration as a police informer was from 2005 until 2009, when she turned on some of her most high-profile clients, including drug kingpin Tony Mokbel and underworld murderer Carl Williams.

The inquiry had been previously told Ashton was notified in April 2006 of Gobbo’s informing. At the time he was heading an independent police watchdog that wanted to call her as a witness in an investigation into the suspected involvement of a former policeman, Paul Dale, in the murders of Terence and Christine Hodson.

Ashton said he could not recall any 2006 meetings with Overland about Gobbo.

He maintained he only learned in 2007, but was shocked to learn the extent of her third round of informing in 2011.

He claimed he knew nothing of her earlier registrations in 1996 and 1999 until the royal commission started.

Counsel assisting the commission, Chris Winneke QC, said it was well known within Victoria police before then, including by assistant commissioner Neil Paterson, who told the inquiry he learned of it in mid-2018.

He said it was extraordinary Ashton was not told, particularly given a high court case involving Gobbo and the force.

Ashton said he became concerned about the extent of Gobbo’s informing in 2011 because of several ongoing court cases, including one involving Mokbel.

But the then deputy commissioner did not notify prosecutors or defence lawyers in those cases, claiming he discharged his obligation by reporting it to his bosses.

Ashton rejected Winneke’s suggestion there were attempts at “keeping a lid” on Gobbo’s informing.

A secret report released earlier this year revealed Ashton once described the opportunity of having Gobbo “potentially solve a bunch of ... murders or prevent others” as “this glittering prize” which could sometimes divert from the sensible steps that should have been taken.

He also defended officers linked to the scandal as doing their duty during the tough period of Melbourne’s gangland wars.

“Do you think that may not be the case now?” Winneke asked.

“What is said in the public discourse about trying to explain to the community what would have been on the officers’ minds at the time, is not to excuse any behaviour,” Ashton said.

“You’re not seeking to excuse any behaviour ... ?” Winneke asked.

“No,” Ashton replied.