Interesting thought about the possible radioactivity from Roxby Downs etc, though having worked there very briefly I don't think there woud be too much radioactive soil sitting around. Maralinga is a different possibility but as mentioned it would be pretty diluted by the time it got here.
As for the weather changing, slightly off topic but was down in Bowral on Sunday where wind chill at 3pm made the apparent temperature -5.3, a slight change from last week. And it felt a lot colder at the top of Mt Gibraltar! On the way home came up the M7 then onto the M4 and there was what looked like a lenticular cloud to our SE, not as well formed as some I have seen in here but had the multiple layers in 2 connected clouds. Unfortunately by the time I got to somewhere I could stop the formation wasn't as visible due to the change of angle so didn't take a photo.