Most of us have done a 'land grab' and bought several different domain names for our business at some time. I just want to clear one thing up right now -
Multiple domains will no longer help with SEO and improving your Google Search Rank (there is one exception as mentioned below).
The biggest danger of having multiple domain names if not done correctly is that Google may treat your site as having duplicate content, and so your search rank can effectively be diluted. If abc.com.au and abc123.com.au both point to the same website, then Google will treat this as 2 separate websites - with duplicate content. In the past having multiple domains worked, but those days are long gone. Honest good content is what works now.
There are, and I'll go through these below -
Here's what Matt Cutts from Google has to say about multiple domain names -
Tell us a little bit about your business and we'll come back to you real soon with an estimate.