If you travelled to Australia and made the popular trip along the eastern coast, you would have memories of a quite mountainous country.
But in fact it is the flattest continent (not the flattest country!) in the world. The eastern coast is the only "really mountainous" area (by Australian standards) thanks to the Great Dividing Range, but even that mountain range is quite old and eroded compared to other, younger mountain ranges in the world.
Famous mountains in other parts of Australia like Ayers Rock, Bungle Bungles, Cradle Mountain and Mt Kosciuszko do not make Australia sound like a flat continent either.
But in fact those "mountains" often are more famous for their beauty than their height. And Australia's highest mountain, Mount Kosciuszko, is only 2228m above sea level.
The absolute majority of the land in Australia consists of endless flat, often treeless plains. Because mountains bring rain, those flat plains get very dry in the lack of rain and support minimal vegetation.
Apart from northern Australia which gets monsoonal rains and the eastern coast where the Great Dividing Range brings some rain, Australia is a very dry continent and it often suffers from droughts.
NOTE: This website is written in British English, which is the English we use in Australia. You will find words like "traveller", "harbour" and "realise", and they are all correct in the language used in Australia.