Does the calculator’s total include GST, or do I need to add it separately?

The calculator has a GST toggle so you can see the estimate both with and without an 18% addition on the subtotal, including contingency. Actual GST treatment on your project depends on how your contract is structured, whether it is a works contract, a material-plus-labour split, or a turnkey agreement, and this should always be confirmed with your accountant and written into the agreement. Do not assume the toggle reflects your exact tax liability until a professional has reviewed your specific contract.