Several ways you can do it.  As Digi said use Replace AU with blank. On your ribbon at top of the spreadsheet click on Find & Replace (on the far right), , from dropdown choose Replace, into the "Find What" enter AU (or AUD)  and in "Replace" nothing leave blank, click replace all.

Alternatively go to Data, highlight the column, click Text to Column, select delimited, Next,  tick space, Finish.  Make sure there is nothing in the next column as your numbers will replace it.