I'm not up to speed on Perth's rainfall pattern, but last time I was visiting I noticed lots more urban sprawl and with it much more concrete cover.
Growing up there, the winter months were a real downer with cold and wet, especially when riding a treadly 5 clicks each way for school.
Later on and working I was seconded to the Met Water Board on the worker gangs and had a bit to do with South Dandalup :- at that time it was predicted it would have a hard time sustaining 25% capacity.
Contrast S.Dandalup at 25% filled and Wivenhoe Dam @ three times that total capacity overflowing and then released on an already drenched Brisbane downstream and you'll get a comparison of what floods really are.
