Two prison inmates have been charged with the attempted murder of drug kingpin Tony Mokbel, who remains in a critical condition in a Melbourne hospital.

CCTV showed Mokbel, 53, being attacked by two men of Pacific Islander appearance in an exercise yard of Victoria's maximum-security Barwon Prison on Monday afternoon.

Police and prison authorities are investigating what sparked the attack, which occurred a day after a newspaper reported Mokbel recently intervened in a standover scheme and extortion racket run by Islander inmates.

Detectives on Tuesday charged two men, both aged 21, over the attack on Mokbel and another prisoner who came to his aid.

The pair have been charged with attempted murder, intentionally causing serious injury and causing serious injury intentionally in circumstances of gross violence.

They will face a filing hearing in the Geelong Magistrates Court on Friday.

Mokbel remains in hospital in a critical condition.

The second victim, aged 31, has been released from Geelong Hospital.

Corrections Commissioner Emma Cassar said Mokbel was expected to survive.

"Nothing that I've been told is leading me to believe it's life-threatening," Dr Cassar told 3AW radio.

Mokbel was walking back to a unit in Barwon's mainstream compound when attacked.

"He was approached by two prisoners who assaulted him, knocked him to the ground and you can see the footage very clearly, showing a number of stabbing actions," Dr Cassar said.

"Another prisoner did come to Tony Mokbel's aid it seems, and he was also injured."

Dr Cassar said there was no indication the second prisoner was Mokbel's bodyguard.

"From the footage it looks like just a random person who was in the area at the time, but there's nothing to suggest that it was a bodyguard or carer of Tony Mokbel," she told ABC radio.

Three homemade weapons were recovered nearby.

Dr Cassar said it is too early to tell if the incident related to a "power play" within the jail, after The Herald Sun reported Mokbel had become a prison enforcer and recently intervened in a standover scheme run by a group of Islanders.

Dr Cassar said the scandal over Mokbel's former barrister being a police informer, which opened a possible window for Mokbel to appeal, would also be considered.

"He was very confident he would get out of jail," Dr Cassar said, adding she suspected it "probably was" irritating other prisoners.

After fleeing Australia while on trial in 2006, Mokbel was captured in Greece in 2007 and was sentenced to a 30-year jail term for drug crimes, with a minimum of at least 22 years before being eligible for parole.

Mokbel was stabbed in the same prison where fellow gangland identity Carl Williams was murdered in April 2010 by another inmate, Matthew Charles Johnson.