Circular 56mm cardboard pin with navy blue background with large gold letter A in the centre and the names of 7 Australian WW1 units around the edge. The cardboard is holed with a tricolour string tied through the hole. The gold A has 1914 to 1918 in black lettering on it. The units around the edge are:
This page on the Australian War Memorial site suggests the badge was a fund-raising item. The units shown are either solely South Australian or have South Australian elements so it’s likely to be South Australian in origins.