How can this be? To give notice or not to give notice?

An employer must have a valid reason for the dismissal of an employee protected from unfair dismissal, although it need not be the reason given to the employee at the time of the dismissal. Shepherd v Felt & Textiles of Australia Ltd (1931) 45 CLR 359 at 373, 377-378…..
However
“Notification of a valid reason for termination must be given to an employee protected from unfair dismissal before the decision is made, Chubb Security Australia Pty Ltd v Thomas Print S2679 at [41]. in explicit terms Crozier v Palazzo Corporation Pty Ltd (2000) 98 IR 137, 151. and in plain and clear terms. In Crozier …… a Full Bench of the Australian Industrial Relations Commission, dealing with a similar provision of the Workplace Relations Act 1996 (Cwth), stated that:
“As a matter of logic procedural fairness would require that an employee be notified of a valid reason for their termination before any decision is taken to terminate their employment in order to provide them with an opportunity to respond to the reason identified. Section 170(3)(b) and (c) would have very little (if any) practical effect if it was sufficient to notify employees and give them an opportunity to respond after a decision had been taken to terminate their employment. Much like shutting the stable door after the horse has bolted.”

And see Adeley v BHP Billiton Iron Ore Pty Ltd (2017) FWC 294 delivered 13 January 2017 per Binet DP