One of Victoria's planned prisons will be doubled in size before it's even built due to a booming population and growing demands on the prison system.

The Ravenhall prison, originally announced as a 500 bed facility, will be expanded to hold up to 1000 offenders as part of a $670-million project.

Premier Denis Napthine says once the prison is built in Melbourne's west it will be on par with the biggest correctional facilities in the country.

"We make no apology for making our community safer," he told reporters on Tuesday.

"We need to have a law and order system that is effective, fair and reasonable ... that requires adequate prison beds."

Dr Napthine said prisoner numbers have grown significantly due to a rise in population over the past two decades.

"This is about community safety. We are about law and order and a fair justice system, so those people that deserve to be in prison can be incarcerated for the protection of people."

He said the prison, which will include 75 mental health beds, will be an important economic driver for the state's western suburbs and will employ 600 workers.

He said when the project was announced it had a 1000-bed "footprint" to ensure it could easily be upgraded if demand required it.

The prison is being delivered through a public-private partnership.

Funding was announced in the state budget for 357 extra prison beds at existing prisons.

Construction will begin in 2015 and is scheduled for completion in 2017.