What's wrong with Drupal? It's modular, very flexible, free, secure, and has been demonstrated to be good enough by other major organisations (ie, the Whitehouse, and Australia is essentially America's lapdog these days).
It's not easy to set up, but, that doesn't make it a poor choice, and what other alternative can you suggest which is proven to be secure, is flexible, modular and has a huge community base?
I hate our government for so many things, but, it's very easy to implement a powerful search engine in Drupal, and there are so many modules available that its a good choice for projects designed to last well into the future.
Also, one of my mates found a serious backdoor in a CMS system used often in Europe (and it was open source). So, since the Whitehorse has likely done some auditing of the Drupal code, it makes sense for the AU government to build on top of their work/testing.