Found this web site though might be worth a read...
http://www.colinandrews.net/Cloud-Ra...2010-0116.html
Extract/quote from the site:
Early Saturday there were sort of concentric circles of echoes between 200 Kms
and 400 Kms from Kalgoorlie-Boulder. However there was no weather around
this area from which radar signals might have been reflected. It would therefore
seem to be due to what is referred to as "anomalous propagation ".(false
echoes) or even possibly dust in the atmosphere.
Regards, Andy, Kalgoorlie-Boulder Met. Office.