As Queensland carries out an inquiry into deadly flooding in that state, the spotlight has turned to other parts of Australia and how well they would be prepared for a catastrophic flood.

The Hawkesbury Nepean Valley in Sydney's outer suburbs has a history of massive floods, and while there has not been a big one for more than a century, plans are in place for such an event.

The suburbs of Penrith and Emu Plains straddle the high banks of the Nepean River in what is a typical picture of Australian suburbia - lots of brick houses, probably built around the 1960s and '70s.

But up to 70,000 people would have to be evacuated if a major flood was to hit the area.

The suburbs sit in a flood plain, and if a flood like the one which hit in 1867 was to occur, the whole area could go under.

Newspaper reports from the time describe an inland sea that destroyed houses, farms and crops and killed at least 13 people. The town of Windsor, downstream from Emu Plains, was submerged.

Steven Molino is principal of Molino Stewart, an environment and natural hazards consultancy that has been working on flood plain management for 20 years.

"The 1867 flood was around 19.5 metres here. So all of these houses would be flooded at least to the eaves, if not higher, in a repeat of the 1867 flood," he said.

He says a major flood today would probably destroy many houses in Emu Plains.

'Hard to comprehend'

Steve Opper, director of community safety with the State Emergency Service, says the Hawkesbury Nepean Valley has a unique shape that can lead to catastrophic flooding.

"The Hawkesbury Nepean Valley is throttled down by a narrow gorge down near what's called Sackville, which is just upstream of Wiseman's Ferry," he said.

"The result of that is that the water can flow into the top of the system very, very rapidly, can't get out, and so you get very dramatic rises in the level of the river.

"So normal river level might be two metres; if you're at the town of Windsor and in the most extreme thought possible, that could rise up to 26 metres, which is a number that's quite hard to comprehend."

That is seven metres higher than the 1867 flood which submerged the valley.

Even if the 1867 flood was repeated, tens of thousands of people would have to be moved.

Mr Opper has designed the evacuation plan for the valley.

"Our contingency planning for evacuation for that valley indicates that we would have to evacuate between 40,000 and 70,000 people just depending on the level of flooding that we're expecting," he said.

"It's a very large number; it would no doubt be probably the largest evacuation of its kind in New South Wales."

Fatal depths

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Alan Ashworth's house in Emu Plains overlooks the Nepean River and is in the firing line. It is a double-storey house set high on his block and seems way above the level of the river below.

But the historic record shows his house could be flooded.

Mr Ashworth says he does not have a detailed flood plan.

"Basically anything downstairs you'd move upstairs. By the time we get water on this section of the road, basically you've lost Richmond, Windsor and all that," he said.

Even so, Mr Opper says moving everything upstairs may not be good enough.

"The difference with this valley is that if people stay there, then the depths the water could get to are almost certainly fatal," he said.

"And so you can't even take an option of saying well, maybe if people don't get out it'll be OK because they'll be able to survive in their house; that's just not an option in this valley."

Because of this, a spillway has been built on the Warragamba Dam upstream from the river and roads have been built and upgraded to help with the evacuation.

These measures will help, but when a big flood comes - and the odds say it will - it will not be stopped.

Mr Molino says the 1867 flood had about a one-in-200 chance of occurrence.

"It can happen. And we have sedimentary evidence from the gorge upstream of Penrith that there's been at least one, if not more floods as large as or larger than a one-in-500 flood in this valley," he said.

"Elsewhere in the country we've had floods of that probability.

"These things do happen. They don't always happen where there's people or houses, but when they do we have a major catastrophe."

This report is the first of a two-part look at Australia's flood planning.

On ABC Radio's PM on Thursday, David Mark will look more broadly at how well Australia is prepared for flooding and the battle between development and nature.