Details
Twix* should certainly state how the amount of the funds to be returned is calculated, and how they got the conversion wrong in the first place. Of course some people may have just trusted that their initial payment was correct and, well, spent it, on maybe a new car, moving house, college fees for the kids (sorry I have no idea how education works 'down under'). So the court case(s) may not actually be that simple.
There is a sort of precedent in the UK. A pensioner was concerned that the then DHSS* was paying her more than she was entitled to and wrote several letters explaining her concerns. The DHSS assured her it was all fine in writing, so eventually she kept the money and spent it. A year or so later, the DHSS discovered their mistake and requested the money back. She refused having (a) a letter from them saying it was her money, and (b) spent it. So the DHSS sued her, and, because she had kept the letters, she won.
Whether the former employees can legally keep the money may depend a lot on what is recorded in writing, and what was agreed between the staff and Twitter at the time.
*Come on! You know I don't mean the sugary comestible.
**The UK's then Department of Health and Social Security, also known facetiously as the Department of Stealth and Total Secrecy, as getting any information out of it was usually impossible.