If it’s a construction contract, you should probably be applying IAS 11 not IAS 18?

In either event, the percentage of completion method is preferred.

I’m not exactly sure what you mean by “completed basis”, but if you are referring to the “completed-contract method” (ASC 605-35-25-88), the alternative method used under US GAAP, this method is not an allowed alternative under either IAS 18 or 11.

Thus, if the outcome of a construction contract cannot be estimated reliably, the IFRS alternative (IAS 11.32) is the zero-profit method.