I am keen to know if a particular network marketing business…

Hi, typical MLM is a legal business structure in Australia as it is in reality a sophisticated sales commission structure.
Whether the MLM business DubLi is legal is another question.
Offering cashback on a purchase is an sales incentive used by many businesses and is perfectly legal.
On the MLM side, what you are describing sounds like a binary system. This is a common structure and it is well-known within the industry, as being a fair and equitable system.
MLM compensation plans are not in themselves illegal - what is important legally in Australia is what induces new participants to part with their money to be entitled to be a part of the compensation plan, and whether the cost of entry is used as the main source of commissions to existing members...
The "dodgy" versions of MLM are configured so that they only work by taking money paid by new participants and paying other participants in commissions, ultimately leading to the financial collapse of the structure if new people stop joining and quite often, eventually, the line of people most recently joined up stand to lose a significant amount of money.
Where commissions are paid from the actual sale of products, then the system is sustainable, and a legitimate business, that is able to pay commissions from "actual sales"... so it is not on a path to financial collapse if no more new members join... (as is the case with the example above).
I trust my explanation makes sense to you.
Kind regards, James.