I always bring Aussie or U.S. cash and convert at a money changer when I need to. The rate is much better doing it this way compared to withdrawing local currency using an Aussie bank ATM card. So depending on the length of the trip I'll take between $1-8k in cash and make sure I don't lose it or get it stolen. I also bring an ATM card as backup with visa debit function but never use it. The HKD is pegged to the USD so take U.S.cash if you have any.