Banks don't lend out that money they source reserves against credit. It's still in Australian dollars. So when you say the bank "borrow" from overseas all you're saying is that companies with AUD need to put money somewhere so they find the best return they can by depositing in the banks.
In a sense it doesn't matter what the interest rate is in the US as you can only buy US treasury securities in USD and the same is true here in Australia.
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