Use the Australian version of Google:
http://www.google.com.au/
Ensure that you've selected 'pages from Australia' and enter your search terms e.g:
plumber sydney
Remember that using 'double quote marks' will group terms together so that Google will only search for that string. e.g. if you type:
melbourne central heating
you'll get all sorts of results about, say 'central melbourne'.
However if you type:
melbourne "central heating"
you'll only get results relating to central heating and Melbourne.
Most web sites will have an email address, so you can contact the companies concerned.
Alternatively, Australia has Yellow Pages, just like ours. You can search the online version here:
http://www.yellowpages.com.au/
Chris
PS: The Ozzie Yellow Pages aren't quite the same as ours. The printed versions include instructions about what to do for snake bites, spider bites, earthquakes, forest fires, etc. It can be quite frightening reading ;-)