The February flood in Lismore has been described as a disaster of "biblical proportions" but modelling shows a worst-case scenario could be as much as two metres higher.
The Wilsons River reached 14.4 metres in the northern New South Wales city, surpassing all other floods on record by more than 2m.
It resulted in multiple deaths, widespread destruction and billions of dollars' worth of damage.
And yet the scale of the disaster could have been much worse according to figures in an independent inquiry that point to a theoretical flood height, known as a Probable Maximum Flood (PMF), of 16-16.5m.
It is a daunting prospect for flood-prone regions that are bracing for a third consecutive La Niña with already sodden soil, creeks and catchments.
What is a Probable Maximum Flood?
A PMF is the largest flood that could conceivably occur at a given location.
Melbourne University professor in hydrology and water resources Rory Nathan says the calculations are based on the highest possible rainfall for a geographical area falling in an already saturated catchment.
"You first start with what's the upper limit of rainfall that can physically be expected at a given location over a given duration," he said.
"You're making some other assumptions about how that rainfall is distributed in time ... and as a result of that you end up with a flood estimate that is pretty well the maximum possible you can expect."
Professor Nathan helped write national guidelines on calculating PMFs and said their most common use was for the design and construction of critical infrastructure, like dams.
He says dams need to be designed to the highest level to guard against failures that could result in considerable damage and loss of life.
"If ever one of our largest dams failed ... the life loss would be very considerable and the environmental damage would be extraordinary," Professor Nathan said.
How often do PMFs occur?
Professor Nathan says the chance of a PMF occurring in any location is "extremely remote".
"In any one year, the likelihood that a PMF would occur is in the order of one in a million," he said.
"They are a theoretical upper limit and I'm not sure anyone could ever say that we've actually had a PMF in Australia."
Professor Nathan said of more concern was the increasing likelihood of record-breaking floods due to climate change.
"Over a hundred-year period, a one-in-a-thousand-year event has a one-in-ten chance of occurring," he said.
"Under climate change, that probability will more likely shift to [a] one-in-four chance."
Should we be planning for a PMF?
While building above a worst-case scenario flood might seem like an easy option, a 2022 report from the Department of Planning and Environment found it was "neither feasible nor socially or economically justifiable" to use PMFs as the basis for floor levels.
In NSW, the more "acceptable risk" is about half a metre above a region's 1-in-100-year flood.
Across the state, there are 366 emergency service buildings – including for police, ambulance, fire and the State Emergency Service (SES) – that sit below this "acceptable-risk" level.
A further 554 are above the 1-in-100-year flood level but below a PMF.
The independent flood inquiry found the 1-in-100-year approach to flood planning was inadequate and recommended a more "risk-based approach" to calculating flood planning levels.
Will Lismore flood again this rainy season?
Former SES deputy director and flood specialist Chas Keys says a third La Niña does not automatically mean there will be more major flooding.
"It means that the dice are loaded in that direction but it's a matter of probabilities not certainties," he said.
"You can get La Niñas with no flooding and you can get El Niño (the opposite, which tend to drought) with floods.
"But you're more likely to get flooding and severe flooding during a La Niña period."
