One way of injecting some competitive tension into the pricing of the variation works is to require the SPV to competitively tender the relevant work.
This is what occurred for Stage 2 of the Gold Coast light rail project, where the SPV ran a competitive tender process for the design and construction of the extension.
Unfortunately, it's generally not possible to apply this approach to the operation or maintenance of the extension, given the need for operations to be integrated with the operations of the SPV's existing Operator.
Getting the SPV to run a competitive tender process for the design and construction of the extension can also be challenging if members of the SPV's original D&C Joint Venture hold significant equity in the SPV.