A Victorian workers’ union has backed reports saying state prisons are dangerous, overcrowded and underfunded.

The Victorian State Government rejects claims that the prison system was not ready for the influx from its “tough on crime” approach in recent years.

This is despite an ombudsmen’s report this week saying that is what happened.

A report on deaths and harm in custody by ombudsman George Brouwer has found overcrowding in prisons is leading to “increasing tensions and violence”.

“The likelihood of prisoners being physically or sexually assaulted or self-harming leading to deaths is greater now than at any time in recent years,” the report says.

The inquiry found a number of prison cells set up for short-term stays are being are being crammed with up to 350 detainees.

“Detainees are being held at the Melbourne Custody Centre for extended periods of time without access to fresh air or natural light in breach of the Victorian Charter of Human Rights and Responsibilities,” the report says.

Karen Batt from the Community and Public Sector Union says clearly the risk for prisoners is high, but there are equally clear flow-on effects for the safety of staff.

“If you take Langi Kal Kal, there's now more than 400 inmates inside there, 100 don't get locked away at night and there is only one officer on at night,” Ms Batt has told the ABC

“Now that is not a situation that I believe is safe and many prison officers have raised these concerns with us, right across the system.

“The sentencing laws have made the prison population unsustainable.”

A spokesperson for Victorian Corrections Minister Ed O'Donohue said the union's claims were untrue.

“We have a higher threshold when it comes to community safety,” Mr O’Donohue himself says.

New prisons are being built, but with several years of lead time the ombudsman says the strain could be overwhelming.

“I am concerned about the likelihood of an increase in the number of prisoner deaths and violent incidents in the meantime,” he said.

The report Deaths and Harm in Custody 2014 is available in PDF form, here.