The terms of senators and members of the House of Representatives are set by the Australian Constitution. Section 7 gives state senators six year terms (with half the number of senators up for election every three years) and section 28 says members have terms of a maximum of three 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 the 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 reflect the wider differences between the two chambers. The two chambers were designed to have different ways of electing their members. The drafters of the Constitution were concerned that if the chambers were too similar they would not be an effective check on each other.