Australian job underpaid me for my position, but need some help regarding fairwork

So I worked for a company in Australia who paid me $4 an hour under what I was supposed to be paid. They have confirmed this but in my contract it states my breaks were paid and rather than the $1500 they owe me they are claiming that they can deduct the paid breaks from this amount owed from underpaying me, can anyone help in regards to whether they are allowed to do this? I've been offered $120 instead of the $1500 offered. Also I'm certain they've taken my breaks as an estimate based on hours worked when I would normally do 14 hour shifts on occasions with no extra breaks as well as other days more than 7 hours were completed and breaks weren't taken for the additional hours.