As usual 404 on the money
( pardon the pun )
NAB will lend up to 80% of valuation if you have something to offset the Currency fluctuation ie: money in an account with them. You can change the currency ( obviously if you change back to AUD you change back to Oz interest rate ) when it fluctuates, but you have to be on the ball. Depending on the size of the mortgage you could be plus or minus 25,000AUD with a .5HKD change. So big money to be gained or lost. If you leave it in HKD, and the AUD drops enough, you will lose a lot more than you gain with the difference in interest rates.
They do seem good to deal with unlike Oz NAB.