"Who was our last senator who became PM?"If your memory goes...

"Who was our last senator who became PM?"

If your memory goes back that far, Liberal Senator John Gorton became the prime minister of Australia on 10/1/1968 while still a senator. He resigned his senate seat on 1/2/1968 to contest a House of representatives seat, the seat of Higgins which he won handsomely.

By convention, the prime minister has to come from the House of Representatives.