Jail operators in Victoria are failing on key safety and security fronts.

Violence is on the rise inside Victoria's male prisons, as inmate numbers increase at Port Phillip's maximum-security facility and a medium-security jail at Fulham.

The state’s auditor-general has found prisoner-on-prisoner assaults are increasing at both facilities.

There were 5022 inmates and 63 reported assaults at the two centres in July 2013, but that number had jumped to 6622 inmates and 133 assaults by June 2017.

“While the rate of assaults per 100 prisoners fluctuates monthly, on average it increased by two per cent per month across all men's prisons,” the report stated for the period mid-2013 to 2017.

Corrections Victoria and prison operators say inmate numbers and the complexity of their needs (such as mental health and drug withdrawal issues) are behind the increase in violence.

Assaults on staff increased from 2015 to early 2017, and were more common at maximum-security prisons.

The report cites a tobacco ban across the system and a large-scale riot at Ravenhall remand centre in June 2015 as a reason for the increase in attacks against staff.

The auditor-general’s report recommends Treasury start providing better cost and benefit advice to government.