Exchanges like bittrex charge about 0.25% for each conversion. You have to do one conversion which is from STEEM or SBD to Bitcoin. Then you need to sell your Bitcoin for AUD which will incur a fee. It's probably best to directly transfer your Bitcoin from exchanges like bittrex to one that pays fiat because every transfer incurs a fee, so if you transfer from exchange to your own bitcoin wallet first and then to one that pays fiat, you're going to have to pay that fee twice. Also, for bitcoin transfers, I think busy times during the week is the best and cheapest time to make transfers.