Interestingly I can't find anything online from a quick search.
Went to Boost Wintergarden in Brisbane literally 20 minutes ago to order a drink, was told today all drinks are free. Any drink on menu and any sizes as well!
Only thing the staff asked me to do is to scan a barcode linking to a donation website, where I can voluntarily donate some money. Staff didn't check if I actually went there nor donated any money. So technically not free but practically achievable.
Didn't ask if its nation wide or how long it lasts, very likely just today.
This is the web site via scanning the QR code