Reserves in eastern Australia are tapping out and reserves in Western Australia are too far away from any city other than maybe Perth (but most of the reserves seem to be in northern Western Australia closer to Darwin). It is more profitable for them to liquify the gas in Darwin and ship it to Japan and China than to build pipes to eastern Australia. In fact, it looks like they are building a liquified NG terminal in southern Australia to get it by boat from northwestern Australia (competing with Japan and china for the same supply).