If less than 20 per cent of the banknote is missing: Full face value is paid.
If between 20 and 80 per cent of the banknote is missing: Value is paid in proportion with the percentage remaining, e.g. $5 value for half of a $10 banknote.
If more than 80 per cent of the banknote is missing: No value is paid.
Can't say I've ever seen Australian currency ripped but good to know. Maybe next time I need to give change to someone, I can just cut the banknote in proportion.
I handle cash at work; quite often I see ripped or taped up notes.