Are you employed on a casual or full-time basis? Did you notify your employer on the day that you were sick and that was why you were not in attendance? Or did you just not attend and then tell them the next day? When determining whether someone's dismissal was 'harsh, unjust or unreasonable, the following factors must, pursuant to s387 of the Fair Work Act be considered, relevant to your matter:

(a) whether there was a valid reason for the termination, in relation to your capacity or conduct, including the effect on the safety and welfare of other employees (b) whether you were notified of that reason (c) whether the employee was given an opportunity to respond e) if termination resulted from unsatisfactory performance by the employee, whether you had been previously warned about unsatisfactory performance.

Arguably if you didn't call in sick, when you reasonably could have, your boss may state that you endangered other employees by not giving them an opportunity to obtain replacement staff for that shift and therefore have a valid reason.

Long answer short, need more info about the situation and your employment contract.