Hi, thanks for your question.
For a bill to be passed into a law, it must be agreed to by both chambers, the Senate and the House of Representatives. Section 23 and 30 of the Australian Constitution explains how the Senate and the House of Representatives vote. In both cases a vote is successful if the majority of members of the chamber vote in favour. In practice most members choose to attend the chamber when votes are taken on important or contentious bills. However, there is no minimum number of members or senators required to attend when a vote is taken, other than the usual rules for a quorum.
A quorum is the minimum number of members or senators required to be in the Senate or House of Representatives at any time. The Senate must have 19 senators present (one quarter of the chamber) to achieve a quorum, while the House of Representatives must have 30 members present (one fifth of the chamber).The President or Speaker will only ask for a quorum if a senator or member asks them to call for a quorum. In practice many bills which have the broad support of members and senators are debated and voted on when the chamber does not have a quorum.