Few guys at work have been discussing this, as they left savings in the UK when returning to australia. Due to the GBP rate, decline they have effectively lost AUD equivalent. They spoke to various people and feedback was it should be a capital gain/loss - but did not get specific confirmation from the tax office. Makes sense that from the time you arrive in australia it is an AUD asset (even though held overseas), and and change in valuation until the point it is physically realised or exchanged into AUD, is subject to tax (could be upside).
On a side note, I believe these boys put the loss through as income, so suspect if investigated they will need to pay it all back.