Thanks for your question.
Under the Australian Constitution, it is up to the House of Representatives to choose which of its members should become the Speaker. There is no rule that states the Speaker must be a government member but this is often the practice. This is because the government is the team that has the support of the majority of members, and the government uses this majority to vote in a member of their own team as Speaker. However, there can be exceptions to this practice. In 2011 Peter Slipper was elected Speaker, despite the fact that he was in the Opposition. He then left his team and became an Independent member while he was Speaker.
It is also important to remember that while cross-bench (Independent and minor party) members are not part of the government team, they still have their own policies and beliefs. Therefore, they may not necessarily be any more impartial than a Speaker from the government team.