Here's a link to some info.
http://www.anbg.gov.au/acacia/species/A-rubida.html
Usually these are pretty bullet proof. I can only guess without seeing but my suspiscion is too wet.
If you have heavy soils (poor drainage) most likely it's drowned.
What is your soil like?
Also, when you dug the hole this is what might have happened if the soils heavy clay etc. The hole becomes a wet well and the plant drowns. If it hasn't been in long you should be able to carefully pull it out of the hole and see the root system.
The opposite problem is the potting mix it came is is slightly hydrophobic and the roots could be tightly matted. Again you need to address with wetting agent and removing from the hole and flaring the roots out.
When planting in heavy soils mounding around the hole is a good idea. I used to say 2/3 root system below grade, 1/3 above and mound soil level with the above part in like a 3' wide dome ... mulch over the top obviously.
Good luck, and some pics would be good.