Read this first, please:
http://forum.migrationhelp.com.au/showthread.php?t=2014
After the withdrawal, you will get a letter from the Department, like this one:
I confirm that the department has received your recent notification that you wish to withdraw your nomination.
This fulfils your obligations in regard to the visa application. As you are no longer a party to the application, Australian privacy law will prevent the Department from discussing any details of the case with you from now on. Please note that this means it will not be possible to tell you the outcome of the application, whether or not the visa applicant is in Australia, or any other information about this particular case, but does not prevent you from supplying information to the Department if you so choose.