You know, I could have sworn that in DNN, if you just enter one Portal Alias, www.example.com, then any request for example.com (no www) would end up on www.example.com .. but now I cant find anything on Google to support this theory! :) 

Is this correct or not, and what does URL Master do in this situation.  Its currently not working for me, typing example.com throws an exception (pageblaster related so I will ask there)

Thanks :)

Yes, DV is right.  You need to add the non-www portal to the list of portal aliases.

There is an option in the Friendly Url Settings page of the Url Master module to 'redirect to single subdomain'.  If you check this box, you can either leave the box blank, for which you will get 'ifinity.com.au', or you can put in www, for which you will get www.ifinity.com.au.

You can check it with my site - try entering ifinity.com.au and you'll get www.ifinity.com.au.