Even though the achievement refers to the first route flown with the Airbus A380, you don't need such an aircraft to get the achievement. As long as the route goes non-stop between Singapore and Sydney, you will get the achievement - so use any aircraft with a minimum range of about 6,300 km (I used the A320-NEO).
Great Circle Mapper route map for reference:
http://www.gcmap.com/mapui?P=SIN-SYD