Parliamentary questions from Greens MP Brad Pettitt have exposed a grim picture of overcrowding in Western Australia's prisons.

Responses to inquiries from Dr Pettitt revealed 62 prisoners at Hakea—most on remand awaiting trial—and 17 at Casuarina are forced to sleep on mattresses on the floor due to severe overcrowding.

"Having people sleeping on mattress on the floor in cells, three people in a cell designed for one, isn't how we should be running our prison system in a rich state," Dr Pettitt said.

The Aboriginal Legal Service WA wrote to Corrective Services Minister Paul Papalia in March, and again in September, asking for urgent action.

Protests were held over conditions in Hakea, also in September.

The National Network, a justice advocacy group, said on Monday it was "appalled" by the revelations.

One Network spokesperson, Tabitha Lean, noted the recent death of a 46 year-old Aboriginal man in Casuarina which, she said, underscored "the urgent need for immediate action".

Reports indicate that Hakea and Casuarina Prisons have nearly 80 cells between them designed for one person but holding three inmates.

"Rather than addressing the systemic causes of this crisis, the WA Department of Justice and Corrective Services have responded by adding more beds to already overfilled facilities," Ms Lean said.

"WA's reliance on prison expansion, with plans for an additional 96 beds across units in Casuarina, signals a stubborn attachment to incarceration over humane, sustainable solutions – WA has a love affair with incarceration and cannot see past the cage."

The over-incarceration of Indigenous people is at its worst in Western Australia, with the over-representation rate roughly four times the national average.

National Network spokesperson Debbie Kilroy asked: "Why is the answer always more mattresses, more beds, and more prison cells?"

"The path forward requires a process of decarceration. It's time to examine who is behind bars and consider other ways of managing this crisis," she said.

"This overcrowding crisis disproportionately affects people on remand who have not been convicted of a crime and could be safely housed in the community. If people are beyond their parole date, let's act swiftly to facilitate their release.

"WA remain dedicated to their inclination toward imprisonment, which resulted in the prison population ballooning by 12 per cent in a year."

A spokesperson for WA's Department of Justice told National Indigenous Times that the Department performs "daily monitoring of the prisoner population and relocates prisoners to ensure facilities are being fully utilised".

"As part of short-term measures to address the population pressures, additional mattresses are being utilised to accommodate three prisoners in some cells at Hakea and Casuarina prisons," they said.

"As of 23 October, there were 59 cells accommodating three prisoners at Hakea and 18 cells at Casuarina."

Ms Kilroy argued that despite the presence of rehabilitative alternatives, "outdated policies that lead to more harm, more deaths, and more instability" are driving up WA's prison population.

"Immediate steps toward decarceration, such as community-based public housing for those on remand and expedited parole for eligible individuals, could reduce pressure on WA's prisons without compromising anyone's safety – it isn't rocket science," she said.

Ms Lean said extended lockdowns, insufficient access to mental health services, limited outdoor time, inadequate sanitation, and lack of privacy contribute to escalating tensions and "a breeding ground for despair, and self-harm".

"The prison system has a responsibility to uphold the basic human dignity of all imprisoned in their cells, and the conditions prisoners are facing right now is a gross abrogation of the State's responsibility to keep their incarcerated citizens safe," she said.

"WA must recognise that it is on a destructive path if it continues to invest in incarceration rather than in community-based solutions."

Ms Kilroy said jurisdictions across Australia must "break this cycle of overcrowding and inhumanity and commit to strategies that offer real support and reduce harm across our communities".