Aerial view of Emu Plains Correctional Centre. Photo: Supplied.

NSW Premier Mike Baird’s plans to build a pop-up prison at Emu Plains housing 400 maximum security inmates in demountables will be built on some of the most dangerous flood plains in NSW, the Opposition has said.

NSW Shadow Minister for Corrections Guy Zangari argued that the state government was aware that developing the Western Sydney site, which already houses a 200-bed women’s prison Emu Correctional Centre, would present “a risk to human life”.

Mr Zangari said that a leaked letter from the Department of Planning to Penrith City Council showed that the government had stepped in to halt an earlier council proposal to develop land adjacent to the pop-up prison site.

“The Baird government’s own planning department recognises that any further development in Emu Plains is dangerous, yet tough talking Minister for Corrections David Elliott is pushing ahead anyway,” Mr Zangari said.

He said Penrith Council wanted to rezone land near the prison for 60 houses but the Department of Planning rejected the planning proposal.

“Given the current evacuation capacity constraints and consequent risk to life, further development of flood prone land in Emu Plains is not supported,” said the Department’s letter to the council.

“Consideration was given to the complex nature of flood evacuation around Emu Plains and that any development of this nature would adversely add to the regional evacuation capacity constraints.”

But a Corrective Services NSW spokeswoman denied lives would be at risk.

“Corrective Services NSW is aware of concerns about the proposed expansion, which includes a 240-bed facility for women and a facility for men,” she said. “If any risk could not be fully addressed then the project would not go ahead.”

Hydrologists will ensure the proposal would have no impact on flooding and evacuation routes will be considered during site investigations.

She said the prison’s expansion would follow state environmental planning legislation, which included exploring potential environmental impacts and taking into account geotechnical information, stormwater and waste management.

The Hawkesbury-Nepean Valley area is well documented as being vulnerable to flooding.

The 2014 Hawkesbury-Nepean Flood Management Review Stage One said that during heavy rain water entered the floodplain much faster than it could escape. Flood waters can rise rapidly and create a ‘bathtub effect caused by natural choke points’.

“A combination of large upstream catchments and narrow downstream sandstone gorges results in floodwaters backing up behind these natural ‘choke points’,” said the review. “Floodwaters rise rapidly causing significant flooding both in terms of area and depth.”

But while the vulnerability of the area to flooding is established the likelihood of a catastrophic flood is up for debate. The last major one was in 1897 when flood waters hit 19.5 metres.

Local flood plain and environmental expert Steven Molino told news.com in 2012 that there was a one in 200 chance of the 1867 flood occurring.

“These things do happen. They don’t always happen where there’s people or houses, but when they do we have a major catastrophe,” Mr Molino said.

The NSW government has also been criticised for a dearth of community consultation about the new prison.

Mr Zangari said: “The government didn’t even have the decency to tell the community about its pop-up prison plans when they first emerged, now they’re learning that their lives are at risk too.”

A NSW Corrections spokeswoman said Justice NSW would commence a targeted community consultation once the development of the concept plan was complete, which would be in the coming weeks.