According to Workplace Authority Australia "Unfair dismissal is when an employer's dismissal of an employee is harsh, unjust or unreasonable. For example, when they fail to give an employee any warning about their workplace performance before dismissing them."

You also need to find out if you are covered by unfair dismissal laws (Workplace Authority Australia 2009: "Employees are covered by federal unfair dismissal laws if they worked for the employer for 6 months or more and their employer has more than 100 employees. The employee must also be either:
• employed by a constitutional corporation (a PTY LTD company)
• employed in Victoria, the Northern Territory or ACT
• employed by the Commonwealth or a Commonwealth authority, or
• a waterside employee, maritime employee or flight crew officer in interstate or overseas trade or commerce.

Note: The number of employees includes part-timers and certain casuals. It also counts the employer's other businesses, such as subsidiary companies."

Detailed about Unfair Dismissal, please visit Workplace Authority Australia.