Hi guys,
First deal so be gentle.
I was going to get the $3 classic burger offer from maccas - I went to a self serve kiosk and added the code manually. At checkout the burger was free.
I've typed in the mymaccas code. The bigmac had a price of 0.00 that was crossed out and had instead 3.00 as new price. I added that big mac. Then added another breakfast meal. Went to checkout and realized the big Mac was free. I deleted the mcmuffin meal and only got the bigmac. Free of charge :)
Edit: Receipt
Mod Edit: Receipt from OP uploaded. Deal was obtained in store at 857am, it may work only at this store or not at all.
Mod Edit 2: OP obtained deal at Wynyard Rail NSW, also reported as working at Chatswood NSW and Melbourne VIC from comments thus far.