Harvey Norman has the biggest range, but if you compare their price list with the identical models from UK online suppliers, such as Dixons, you might notice that Australian prices are often more than double those in the UK.
Some, but not all, radios from the UK will work in Australia. We use the new DAB+ standard, whereas the UK runs on the older DAB. If the radio contains sufficiently modern innards then it can be upgraded to DAB+ by the application of a free firmware update. If you're going to import a radio from the UK then you must check this out in advance.
Hopefully, over time Australian digital radio prices will stabilise at a level more consistent with the UK. Until then I doubt the benefits of digital radio, such as less interference and more stations, will outweigh the pricing issues for the masses.
Update 31-May: Australian Personal Computer has a nicely comprehensive report on the launch of digital radio.