This sign was created in 1965 by construction crews sealing the Eyre highway across the Nullarbor. The diamond shape measures 3.2km across (2 miles) by 1.6km high (1 mile). Inside the diamond, the word 'Readymix' (now overgrown and not visible) was listed in the Guiness book of records between 1972-1992 as the largest lettering in the world. The sign was actually used as an air navigation aid for many years.

Here is an image of how it looked a few years after it was made.
Unfortunatley, being a newbie here, I'm not sure how to make an overlay.

Here is an overlay using your JPG file.

i remeber seeing tha while me and my family were driving to perth ages ago, thought it was some kind of crop circle

I must have spent a couple of days trying to find this little remembered bit of Australian history. Many hours of searching the web gave little clues, but eventually I found it . But I am a lot more educated about so many things I never realised were interesting such as the incredible network of caves that extend under the Nullabor.