Market value, or "total cash"?
If you're just talking about market value, then you probably don't need to - unless you're really, really enthusiastic about hedging your translation risk.
If you've got "total cash" balances in those currencies, then just do a common-or-garden-variety FX trade, but set the destination to FXCONV instead of IDEALPRO. Generally the software is smart enough to know where you want to route your trade to, though.
If you're using TWS, you can just pull up your account window, right-click on the AUD currency line, and select Close Currency Balance - it'll automatically create an FX trade for the relevant amount, and all you have to do is click Transmit.