There are basically 2 offences created by the Legislation. 
They cover both payment and receipt.

The overriding requirement of the first offence is that the benefit is provided dishonestly.

"536D Giving, receiving or soliciting a corrupting benefit 
Giving a corrupting benefit 
(1) A person (the defendant ) commits an offence if: 
(a) the defendant dishonestly : 
(i) provides a benefit to another person"

Dishonestly requires an objective assessment and that the person providing it knows what that standard is.

"536CA Dishonesty 
(1) For the purposes of this Part, dishonest means: 
(a) dishonest according to the standards of ordinary people; and 
(b) known by the defendant to be dishonest according to the standards of ordinary people. 
(2) In a prosecution for an offence against this Part, the determination of dishonesty is a matter for the trier of fact"

Good luck with getting 12 jurors to agree on all but the most obvious cases of this.

I would think most Union negotiators would feel (subjectively) justified in demanding benefits for the Union in their deals and would be “aghast” at any suggestion that it would influence them.

There could be situations where an employer makes a payment with the required intention but the Union Representative doesn’t receive it with that intention or is able to maintain that they were unaware of the intention of the employer. See S 536 D (2). ie the employer would commit an offence but not the recipient.

The “dishonest” benefit referred to in Section 536 D (1) has to be provided with a required intention.
"and 
(b) the defendant does so with the intention of influencing a registered organisations officer or employee (who may be the other person): 
(i) in the performance of his or her duties or functions as such an officer or employee; or 
(ii) in the exercise of his or her powers or performance of his or her functions under this Act or the Registered Organisations"
OR
It has to not be provided “Legitimately”.
"(iii) to give an advantage of any kind in connection with the relevant affairs, which would not be legitimately due, to the defendant, a spouse (within the meaning of the Registered Organisations Act) or associated entity of the defendant, or a person who has a prescribed connection with the defendant.
(4) In a prosecution for an offence against subsection (1) or (2), the determination of whether an advantage would not be legitimately due is a matter for the trier of fact."
The same comment re the Jury applies.
"(6) In working out whether an advantage would not be legitimately due to a person, disregard
(a) whether the advantage might be, or be perceived to be, customary, necessary or required in the situation; and 
(b) the value of the advantage; and 
(c) any official tolerance of the advantage"

They are told what to disregard but given no indication of how” legitimate” is to be judged.
I would think it would be approached by considering whether “that’s fair enough” and it is obviously connected with the initial objective assessment of “dishonesty”.
Again might be difficult to reach any agreement about that.

In addition to the offence under S536 D which is outlined above there is a further offence under S536 F relating to the payment of a benefit. A similar offence is created for the receipt under S536 G.
This is then subject to a number of exclusions.
The exclusions cover various items including entertainment and travel and Membership fees but the main scope for manoeuvre would come with the requirement for a payment to be for the Dominant purpose of benefiting employees.
The use of “dominant” in conjunction with “sole” would I think require some type of percentage judgement and not merely an assessment of the most important reason amongst many but it isn’t difficult to see that it provides plenty of room for other purposes.
"536F Giving a cash or in kind payment 
Giving a cash or in kind payment 
(1) A person (the defendant ) commits an offence if: 
(a) the defendant is a national system employer other than an employee organisation; and 
(b) the defendant: 
(i) provides a cash or in kind payment to another person; or 
(ii) causes a cash or in kind payment to be provided to another person; or 
(iii) offers to provide, or promises to provide, a cash or in kind payment to another person; or 
(iv) causes an offer of the provision of a cash or in kind payment, or a promise of the provision of a cash or in kind payment, to be made to another person; and 
(c) the other person is an employee organisation or a prohibited beneficiary in relation to an employee organisation; and 
(d) the defendant, a spouse (within the meaning of the Registered Organisations Act) or associated entity of the defendant, or a person who has a prescribed connection with the defendant, employs a person who is, or is entitled to be, a member of the organisation and whose industrial interests the organisation is entitled to represent.

3) Subsection (1) does not apply to the following cash or in kind payments: 
(a) a payment to the organisation: 
(i) made by deduction from the wages of an employee of the defendant who has agreed in writing to become a member of the organisation; and 
(ii) made for a membership fee payable by the employee; 
(b) a benefit provided and used for the sole or dominant purpose of benefiting the defendant's employees, or the defendant's former employees in relation to their former employment; 
(c) a gift or contribution deductible under section 30-15 of the Income Tax Assessment Act 1997 and used in accordance with the law; 
(ca) a benefit of nominal value (meaning no more than 2 penalty unit s) associated with travel or hospitality during consultation, negotiat ion or bargaining; 
(cb) a benefit of nominal value (meaning no more than 2 penalty units) that is: 
(i) a token gift, an event invitation or a similar benefit; and 
(ii) given in accordance with common courteous practice among employers and organisations; 
(d) a payment made, at no more than market value, for goods or services supplied to the defendant in the ordinary course of the organisation's business; 
(e) a payment made under or in accordance with a law of the Commonwealth, or a law of a State or Territory; 
(f) a benefit provided in accordance with an order, judgment or award of a court or tribunal, or in settlement of a matter before the FWC or a genuine legal dispute ; 
(g) a non-corrupting benefit prescribed by, or provided in circumstances prescribed by, the regulations. 
Note: A defendant bears an evidential burden in relation to the matters in this subsection (see subsection 13.3(3) of the Criminal Code ). "