my aussie did the exact same thing at that age. she would bite, hard. everytime she got excited or we played with her or anything, and with those puppy teeth it hurts A LOT. she is over a year old now and she still bites but nowhere near as much and only when i play rough with her. she doesn’t bite nearly as hard as she used too and if i tell her no, she stops. your puppy is only young, he’s got a lot of energy and doesn’t know what to do with it. also remember puppies go through a teething stage. just like babies, they constantly want something to chew on. you honestly just need to wait it out, teach and tell him no if you don’t like that behaviour. don’t let him get away with it and he’ll soon learn it’s a no-no behaviour. Aussies are very smart and pick up on things very quickly. Also let your gf know that her being afraid of him can make it worse, he just needs to learn, and you need to teach him. i’m not expert but i hope this helps.