If your project has a well-defined scope of work, a lump sum contract is a straightforward solution since contractors should be able to accurately estimate costs. If the project scope is unclear, contractors can bid with time and materials contracts to mitigate risk.
AS2124 is generally considered a contract involving risk for the Contractor and the Project's Owner however, when amended, the risk can swing in either direction. The parties to AS2124 are. The Principal (the party commissioning the construction project, i.e. the Project's Owner);
Overall, while AS 2124 primarily focuses on arbitration, AS 4000 provides more flexibility in choosing dispute resolution methods, and AS 4300 relies on the provisions of AS 4000 for resolving disputes.
Both involve the meeting of minds and exchange of promises, but a contract typically entails a more formalized arrangement, often documented in writing, and carries legal enforceability. Conversely, an agreement can be informal and may not always be legally binding.