After the recent floods in south-east Queensland were labelled a one-in-100 year event, another such natural disaster shouldn't happen for 99 years, right? Wrong.
Similar terms were applied to the 2011 Brisbane floods and some quick maths will reveal that was only 11 years ago.
Maryborough saw devastating rainfall that flooded the central business district at the start of January, only for a repeat occurrence in late February.
So how did flooding occur in 2011 then again in 2022 and how much does it have to do with climate change?
We've spoken to researchers to break down the mathematical probability of this oft-quoted expression.
What is a one-in-100-year weather event?
Matthew Mason from the University of Queensland's school of civil engineering explained it as an event of a certain magnitude, like major flooding, that occurs on average once every 100 years.
"That is, if we had a period of 1,000 years you would expect to see 10 events that were of, or exceeded, the magnitude of the one-in-100-year event during this period," Dr Mason said.
"It does not mean that you will only see one such event within any given 100-year period.
"In fact, doing the mathematics on it, if we assume that the process causing the extreme event is stationary with time (that is, factors such as climate change are not messing with the likelihood of events), then we only have approximately 37 per cent chance that exactly one event will occur during a randomly chosen 100-year period.
Environmental historian Margaret Cook from the University of the Sunshine Coast said the origin of the phrase with regard to floods dates back to the US in the 1930s and was adopted in Australia in the 1970s.
A major flood was deemed as having a one per cent chance of re-occurring in 100 years.
"This meant that there is a one per cent chance that a flood of that magnitude (or larger) will happen in any given year," Dr Cook said.
"For a home in a 100-year flood zone, the actual probability of experiencing a flood of this magnitude is greater than a 26 per cent chance in 30 years, a 75 per cent chance in 100."
She said as terminology moved from the technical world to the public, subtle changes in wording created massive changes in meaning.
"The popular shorthand for a flood that a hydrologist would describe as having a 100-year recurrence interval, became a one-in-100 year event," she said.
"This led to the misunderstanding that this implied a flood of that magnitude would not happen for 100 years."
So why did the 2011 and 2022 floods happen in such close proximity?
Dr Mason said there had been a less than 1 per cent chance of the two events occurring.
This makes it highly unlikely but not impossible.
"However, a caveat on the above point is that the 2011 and 2022 events were not the same things, so are not both strictly one-in-100-year events."
Are one-in-100-year weather events happening more often?
In short, yes, Dr Mason said.
"However, what needs to be kept in mind is that disasters occur because of the combination of the hazard (wind, rain, rising water levels in rivers), the exposure (how many people or things are in the way of any of the aforementioned hazards) and vulnerability (how people or infrastructure responds to hazards).
"With time, exposure is always increasing. If population increases, so too does the number of buildings and infrastructure that we have.
"So, all else being equal, the impact that hazards have will increase purely because there is more stuff to be damaged (larger disasters) or there are now people and infrastructure in the way of hazards that have historically not any significant impact (more frequent disasters)."
How much is this to do with climate change?
Dr Cook said there is no question climate change is contributing to the frequency and severity of these weather events.
She noted that climate change is expected to increase severe weather events over the next 80 years.
"Climate modelling anticipates that a 1-in-20-year annual maximum 24-hour precipitation rate is likely to become a 1-in-5 to 1-in-15-year event by the end of the 21st century in many regions," she said.
"While the frequency of tropical cyclones may decline or stay constant as the climate warms, the number of severe tropical cyclones is expected to increase."
Does using the expression cause confusion?
To put it bluntly, yes. So much so, Dr Cook said the terminology used by authorities to communicate to the public in natural disasters needs to change.
"I think the time may have come to abandon terms like 'one-in-100'. I think they add more confusion than clarity," Dr Cook said.
She and Dr Mason agreed there is a high degree of difficulty in predicting one-in-100 year events and conveying the probability of their occurrence to the general public.
"It's probably still the case that if you tell people that there is a 1 per cent chance of something occurring, they will still likely dismiss it as something that won't happen," she said.
"So, in many ways I think we end up back in the same place — people largely disregarding that these events can and do occur."
Dr Cook said the expression was never meant to be used as an absolute, which is how many people perceive it.
"Flood plain dwellers do not want to be given a probability of a flood but want a simple guarantee of flood immunity," she said.
"The belief that a similar flood will not occur for 99 years cultivates a misplaced certainty of occurrence and a misguided sense of security."
How can I protect myself from these rare events?
An Insurance Council of Australia spokesperson told the ABC there are no homes in Australia that cannot be insured against floods, though obviously the cost of that coverage can vary enormously.
"Flood cover is a standard policy inclusion except where it is expressly stated cover for flood is excluded," the spokesperson said.
"At present, there is no area of Australia that is uninsurable, although there are some locations where there are clearly affordability and availability concerns.
"Insurance prices risk, and that means that for those in flood-prone locations, flood cover can be costly.
"That is why the Insurance Council has called on all Australian governments to do more to protect homes, businesses and communities from the impacts of extreme weather."
