Information on contracts with a value of $50 000 or more, awarded by agencies, is available from the Tenders website and also from each agency's annual report. The information on the Tenders website includes the expiry date for the contract. This provides you with the opportunity to approach the relevant agency and make yourself known prior to a new procurement process being run for any new contract.
For whole-of-government / common-use contractors (ie those that are used by all agencies) refer to the information on common-use contracts. This section of the website details the specific contracts that agencies are required to use, and includes information on the expiry date for each of those.