Which of the following is australia's most important and well-known natural wonder

I would say the Great Barrier Reef

Sydney Harbour is one of the most well known natural wonders in Australia and quite possibly the most recognisable and beautiful harbour in the world. The Sydney Harbour stretches 20km inland and covers approximately 240 kilometres of shoreline; the area has a large number of beaches (including the iconic Bondi Beach) and inlets tucked away; there are also large sections of parklands, reserves and gardens. Sydney Harbour is home to the world famous Sydney Opera House and Sydney Harbour Bridge.