Worse, a number of ISPs use Telstra's lines, eg. iiNet, Internode, TPG. I have no idea if that agreement also covered others using the same cables.
(Telstra was government-owned until it was privatised from 1997-2000, and as such has a monopoly on most of the phone lines; the government sets the wholesale rates at which the lines are leased to other companies. Optus ran a bunch of their own cables, but the coverage isn't nearly as extensive.)