"What happens to a law that is not passed by one of the houses?"

Great question George! Both the House of Representatives and the Senate have to agree to a bill (a proposal for a new law or a change to an old one) before it becomes a law. If one of the houses doesn't pass the bill, it cannot become a law.

If this happens the originating house (the house in which the bill is introduced) may;

Section 57 of the Australian Constitution provides a procedure if the House of Representatives and Senate cannot agree on a bill introduced in the House of Representatives. If the Senate and the House of Representatives can not agree on a bill introduced twice in the House of Representatives (and all the requirements in the Australian Constitution have been met), the Prime Minister can ask the Governor-General to call an election for all the seats in the House of Representatives and the Senate. This is called a double dissolution and has only occurred seven times.

Check out this link for more information about double dissolutions and what can happen if the bill is still not passed by the Senate after the election.

diagram to help explain What happens to a law that is not passed by one of the houses?