I often read in various posts and blogs of law firms that
(a) an employer is entitled to rely upon grounds to justify the dismissal of an employee which are ascertained after the dismissal. This is correct.
(b) an employer is not permitted to rely upon grounds for dismissal which are different from those used at the pint of dismissal to justify it to the employee. This is incorrect.
Facts justifying dismissal, which existed at the time of the dismissal, should be considered, even if the employer was unaware of those facts and did not rely on them at the time of dismissal. Shepherd v Felt & Textiles of Australia Ltd  HCA 21 (4 June 1931), [(1931) 45 CLR 359 at pp. 373, 377‒378].
Facts which existed at the time of the dismissal, but came to light after the dismissal may:
- justify the dismissal when it would otherwise be harsh, unjust or unreasonable, or
- render the dismissal harsh, unjust or unreasonable. Australia Meat Holdings Pty Ltd v McLauchlan Print Q1625 (AIRCFB, Ross VP, Polites SDP, Hoffman C, 5 June 1998), [(1998) 84 IR 1 at p. 14]. See also Dundovich v P & O Ports PR923358 (AIRCFB, Ross VP, Hamilton DP, Eames C, 8 October 2002) at para. 79.
Ultimately, the Commission is bound to determine whether, on the evidence provided, facts existed at the time of termination that justified the dismissal. Lane v Arrowcrest (1990) 27 FCR 427, 456; cited with approval in Byrne v Australian Airlines Ltd  HCA 24 (11 October 1995) at paras 131, 136 (McHugh and Gummow JJ), [(1995) 185 CLR 410 at pp. 467, 468].
The reason for the termination need not be that which was given by the employer. It can be any reason underpinned by the evidence provided to the Commission. MM Cables (A Division of Metal Manufacturers Limited) v Zammit Print S8106 (AIRCFB, Ross VP, Drake SDP, Lawson C, 17 July 2000) at para. 42. See also Fenton v Swan Hill Aboriginal Co-operative Ltd  FCA 1613 (4 September 1998).
If the employer seeks to rely on a reason for dismissal other than the reason given or relied upon at the time of the dismissal ‘they will have to contend with the consequences of not giving the employee an opportunity to respond to such reason’. Fair Work Act ss.387(b) and 387(c). See also APS Group (Placements) Pty Ltd v O’Loughlin  FWAFB 5230 (Lawler VP, O’Callaghan SDP, Roberts C, 8 August 2011) at para. 51, [(2011) 209 IR 351].