A 1% Annual Exeedance Probability Flood, also known 100 year ARI flood, means you have a 1 in 100 chance that a flood of that size or larger could occur in any one year. It does not mean that if it floods one year, it will not flood again for the next 99 years.
The 100 year flood terminology is misleading, hence the preference for using the 1% AEP or 1 in 100 annual chance flood.
Some parts of Australia have received a couple of 100-year floods in one decade. The probability of experiencing a 1% AEP flood in a particular location over an 80 year period is about 55%, and the chance of experiencing two is around 19%.
The 1% AEP flood level is generally used as the Design Flood Level for most types of development. The extent is the area below this level that will be impacted by a flood of this magnitude.