This is a really interesting question Peter.
Section 35 of the Australian Constitution states the Speaker of the House of Representatives must be a member of the House of Representatives and be chosen by the other members of the House to be their spokesperson and chair its meetings.
Appointing a current judge to the position of Speaker would also be against the principle of the separation of powers. This principle proposes that the power to govern should be divided between different bodies to avoid any one branch having too much the power. In Australia power is divided between a law-making body (parliament), an executive body (executive government) and courts (judiciary). Having a judge as the Speaker of the House of Representatives would lead to the judiciary having law-making power.