Here is an extract from a recent decision of the Full Federal Court which contains a good legal summary of the evidence and proof issues in an adverse action claim.
“THE ADVERSE ACTION CLAIM
The relevant provisions of the Fair Work Act
28 Section 340(1) of the FW Act precludes “adverse action” being taken against another person because, among other things, that person has exercised, or purported to exercise, a workplace right. It provides (relevantly):
(1) A person must not take adverse action against another person:
(a) because the other person:
(i) has a workplace right; or
(ii) has, or has not, exercised a workplace right; or
(iii) proposes or proposes not to, or has at any time proposed or proposed not to, exercise a workplace right; or
(b) to prevent the exercise of a workplace right by the other person.
Note: This subsection is a civil remedy provision (see Part 4-1).
29 Section 341 defines the circumstances in which a person has a workplace right:
(1) A person has a workplace right if the person:
(c) is able to make a complaint or inquiry:
(i) to a person or body having the capacity under a workplace law to seek compliance with that law or a workplace instrument; or
(ii) if the person is an employee—in relation to his or her employment.
30 By s 342(1) of the FW Act, an employer takes adverse action against an employee if (relevantly) the employer dismisses the employee.
31 Sections 360 and 361 facilitate proof by an applicant of a claim of adverse action. Section 360 provides that, for the purposes of Pt 3-1 of the FW Act, “a person takes actions for a particular reason if the reasons for the action include that reason”.
32 Section 361(1) creates a rebuttable presumption:
(a) in an application in relation to a contravention of this Part, it is alleged that a person took, or is taking, action for a particular reason or with a particular intent; and
(b) taking that action for that reason or with that intent would constitute a contravention of this Part;
it is presumed that the action was, or is being, taken for that reason or with that intent, unless the person proves otherwise.
33 Section 361 thus throws onto respondents the onus of proving that which is peculiarly within their knowledge. See, eg, Board of Bendigo Regional Institute of Technical and Further Education v Barclay (2012) 248 CLR 500 at  (French CJ and Crennan J), citing General Motors-Holden’s Pty Ltd v Bowling (1976) 51 ALJR 235 at 241.
34 Whether a respondent has discharged the “reverse onus of proof” is a question to be resolved at the end of a proceeding, and upon consideration of the entirety of the evidence adduced. As Jessup J (Rangiah J agreeing) explained in Construction, Forestry, Mining and Energy Union v Anglo Coal (Dawson Services) Pty Ltd (2015) 238 FCR 273 at :
27 In the context of a provision such as ss 340 and 352, the effect of s 361 is to reverse the legal onus in relation to the reason or reasons for which the adverse action was taken. That is to say, at the end of the trial of fact, the question will be whether the respondent has established, on the civil standard, that the action taken was not taken for a reason, or for reasons which included a reason, proscribed by the legislation. That question is to be answered by reference to all of the evidence which bears upon it.
35 It follows that the party making an allegation that adverse action was taken “because” of a particular circumstance must establish, in light of all the facts established in the proceeding, the existence of the circumstance as an objective fact. See, eg, Tattsbet v Morrow (2015) 233 FCR 46 at 75  (Jessup J, Allsop CJ and White J agreeing); Australian Building and Construction Commissioner v Hall (2018) 261 FCR 347 at 380  (Tracey, Reeves and Bromwich JJ); Board of Bendigo Regional Institute of Technical and Further Education v Barclay (2012) 248 CLR 500 at 523-524  and  (French CJ and Crennan J).”
TechnologyOne Limited v Roohizadegan  FCAFC 137 delivered 5 August 2021 per RANGIAH, WHITE AND O’CALLAGHAN JJ