Hi AgentMail,
The Copyright Council states:
"Generally, it will be an infringement if someone uses a substantial part of copyright material, without the permission of the copyright owner, in a way exclusively reserved to the copyright owner (unless there is a special exception for you to use)."
One of the 'ways exclusive reserved to the copyright owner' in the case of written material such as articles is the right to reproduce it and communicate it to the public. - and, unfortunately being unable to contact the owner/author does not necessarily mean you can go ahead and do it.
If you visit
www.copyright.org.au you can download fact sheets that set out a lot of information and examples of when you do/don't need permission etc. (I tried to attach the specific one to this post but the file's too large).
If you go specifically to:
http://www.copyright.org.au/find-an-answer - you can search for fact sheets, but they also have several 'main' ones just sitting on that particular page including about infringement/when you need permission etc.
Morally, if you are able to obtain permission you will most likely need to attribute the author appropriately.
If the site is a .com.au you can search it's deatils at
www.ausregistry.com.au and you may be able to obtain an email address rather than having to try to contact via the site; if not a .com.au there are several sites that will allow 'whois' look ups that might give you this information.
Hope it helps!