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if you have $5000 sitting in credit on the card it earns no interest. if you have that same money in a high interest e-saver it earns about .89c a day.
if you withdraw say $300 while you are overseas, that will cost you 20c in interest overnight (at 20% interest).
you should not need to pay anymore than one days interest as you can transfer the money from your e-saver almost immediately to cover the withdrawal. I usually to it next day so that the transfer hits after the withdrawal and covers it.
the other issue is that if you have $5000 and make purchases those come out of the $5000 as does your cash advance.
if you withdraw (without a credit balance) then any money you top up goes immediately to pay of the cash advance. the purchases sit there earning no interest anyway.
over a month, using my system gains about $27 in interest in my saving account, but the interest charges on the credit card (assuming you took out $300 a day) would only be in the region of $6.20...