According to the wiki HSBC charges a 1% conversion fee for all overseas withdrawals and at least $1/transaction for withdrawals from non-HSBC ATMs. So presumably you would only be charged the 1% if you used your card at HSBC-owned ATMs. But you should verify with your local bank before you travel.

There are plenty of smaller banks that charge 0-1% for withdrawals at any compatible ATM overseas, however. So I wouldn't open an HSBC account just because there are HSBC branches in Oz (although if you're traveling there regularly, it certainly wouldn't hurt to have as a backup).