A lot of domains are already registered with no real website associated with them. Its not enough to say whether a domain is available by typing in the URL and checking for an error message.
You can check .com, .net and .org by going to http://www.betterwhois.com/
You can check .com.au by using: http://whois.ausregistry.net.au/
If you are registering for a .com.au domain, then have the following information handy
Confirmation that the domain name is a close and substantial connection to the legal entity, or that the domain name is an exact match, acronym or abbreviation of the legal entity.
Entity or Company Type (e.g. Company, Partnership, Individual, Sole Trader, Association)
Entity or Company Name
Company Number (ABN, ACN, RBN or Association Number)
State of Registration (NSW, VIC, QLD, etc..)
If a domain is about to expire you have a chance of being the new owner. Its not as simple as it once was. Here is an article about how to snatch an expiring domain
nominet manage the .uk domain space. If you own a .uk domain you will have an account with them. A owner change must be done via nominet. This includes updating your address details.