Checking a pictorial chart depicting annual rainfall from 1890...

Checking a pictorial chart depicting annual rainfall from 1890 until present, there is nothing to indicate that there is anything different for the south west region of Western Australia from the mid 70's onwards in magnitude or duration that hasn't been experienced before.
I am not sure that the mid 70's is the correct starting point for such comparisons, and there are many that do refer back to that point. The reason being that the years 1973 to 1976 were exceptionally wet years over the entire continent with 1973 being above average rains over nearly all parts and slightly less of the continent continuing to receive above average rains for the other years.
If they picked an exceptionally dry period to refer back to they would have to go back to perhaps to the 1930's to find a period as dry as what the mid 70's were wet.