Senate estimates is when senators scrutinise – closely examine – how the government is spending taxpayers' money. The estimates committees question ministers and government departments on how they have spent this money and what they plan to spend it on in the future.
The 8 standing – permanent – committees of the Senate conduct the estimates hearings. Each has government departments and agencies to examine in the estimates process. For example, the Community Affairs Committee questions the Department of Health and Services Australia, and the Legal and Constitutional Affairs Committee examines the Attorney-General's Department and the Department of Home Affairs.
This fact sheet has more information about the Senate estimates process, or you could have a look at this detailed Senate Brief on the work of Senate committees in considering estimates.