The terms of senators and members of the House of Representatives are set by the Australian Constitution. Section 7 gives state senators 6 year terms (with half the number of senators up for election every 3 years) and section 28 says members have terms of a maximum of 3 years. Territory senators have the same terms as members. This means that senators have a term that is double or—in the case of an early election for the House of Representatives—more than double that of members.
The drafters of the Constitution were inspired by the United States Senate when deciding how our Senate would work. Six year terms and the rotation of senators in the Australian Senate were taken from the model of the United States Senate.
The difference in the terms of senators and members of House of Representatives reflects the differences between the 2 chambers. Senators represent the whole of their state, a much larger number of people than members of the House of Representatives. A 6 year term gives them more time work for the people they represent.