It depends on what quantity she's importing, and how it's being shipped.
On a commercial carrier (i.e. Fedex, etc.) there is a limit of AUD$250 (i.e. if the goods have a declared value over that, you need to pay tax to customs). But for USPS handing over to Australia Post, the limit is supposed to be higher (AUD $1000).
Customs home page would probably be the place to find out more information on this.
I am not aware of any import restrictions from the US to Australia. This doesn't mean there
aren't any, but I do import from time to time and I'm not
aware of any.
Theoretically it would happen like this: you would go to your shipping company, you would fill out the appropriate customs form, and you would declare the value of the goods. They'd get shipped over here. Then one of two things will happen: 1) The goods will be released to the customer, and they'll never hear from customs, or 2) She'll get a bill from customs ($50 + 10% of the value of the goods
if I remember correctly), she'll pay it, the goods will be released to her.
Either way, you don't need to do anything your end (other than the initial standard shipment) and it's her problem.
I can tell you from experience that customs in Australia lets a lot through. For example, a friend of mine bought a $2000 camera lens that should have required a duty payment, but never had to make one. Presumably it was his responsibility to declare it after the fact, but that's a whole other matter.
That's what all this tends to come back to: it
might cause problems for your client, but that's not very likely, and they shouldn't be large ones. There is a chance there will be more to pay when the goods hit the country though. But basically they send you a bill, it's not like you get in trouble.
Thing is, a box of wholesale t-shirts B2B to sell in a retail shop is not the same as ordering a container load of blanks from China. I can't promise you won't have a problem, but I've ordered cases of blanks from the US (e.g. from Alternative Apparel) without a problem.
I would recommend taking a look at sites like the one above and not taking my word for it. It shouldn't be a scary or impractical process though.
Importing Shirts in to Australia 
