The Privacy Act merely regulates the way individuals’ personal information is handled by a company - particularly how they collect, store and use personal information. It isn't really designed to apply to situations like this. The Privacy Act 1988 will apply to public employers and large private sector employers - and its provisions may apply to employee emails that include personal information, however it does not apply to workplace surveillance relating directly to information regarding the employee’s position with the company. If the employer is a small private company - the Privacy Act will not apply at all.
When it comes to individuals rights to privacy, there is no law or principal conferring any prima facie "right" to privacy. In the situation you have described above, I would think that since it is the employer's computer and premises where the activity occurred, the employer would be free to use the information that they had found. It may be different if you had intentionally accessed password protected information, or intercepted the messages somehow electronically, however this does not appear to be the case. Employers are permitted to monitor staff emails that are sent on work computers so in this situation I don't see why you couldn't use information that was left on the screen of an employee's computer.
I am interested to hear opinions from other forum members.