Is that the weed with the pointy green balls? I spent a lot of time with a pair of pilers getting those out of my boots so I wouldn't get them quarantined when I came back to the US.
Here is some info on it. It says that sheep (jumbucks) eat it, but they can get sick if they eat too much.
http://www.agric.wa.gov.au/content/P...FN042_2003.PDF
The American kind in Texas and the southwest US is also unhealthy and a problem weed.
http://texnat.tamu.edu/cmplants/toxi...%20caltrop.pdf