That funding was confirmed, with bipartisan support from Labor, after Mr Turnbull’s ouster as prime minister. Scott Morrison launched the project at a November event attended by Victoria Cross recipient Ben Roberts-Smith, and business and cultural leaders.

The new documents were obtained by Heritage Guardians, a community campaign opposed to the expansion, and show Mr Stokes guaranteed a forecast price tag of $498.2 million over a nine-year period.

Outgoing director Brendan Nelson told a budget update meeting that a $7 million underspend would go to other parts of the project.

Project director Tim Wise told the meeting “that commencing work early may assist in escalation cost savings later in the project,” the documents show.

“[Brendan Nelson] noted that the chairman had given his personal guarantee to the Prime Minister that the Memorial would only seek $500 million.”

A detailed business case for the project was delivered to the federal government on December 21.

Heritage Guardians spokesman David Stephens questioned why a personal guarantee from Mr Stokes was needed, unless there was concerns the budget could be “somewhat rubbery”.

“It’s unclear, though, whether the guarantee bound just Mr Stokes personally or Mr Stokes on behalf of the Memorial Council,” he said.

“As to the processes generally, the committee minutes give the strong impression that the funding of the project and the development of the detailed business case for it ran on separate tracks.

“The Department of Finance rules for such projects, on the other hand, strongly suggest there is a connection between the two processes.”

In February, The Australian Financial Review revealed Mr Stokes donated more than $700,000 to pay for the project’s lavish launch at Parliament House.

An $842,000 contract for a stakeholder function prompted Dr Nelson to confirm Mr Stokes had given permission for his $740,000 donation to be revealed publicly.

The Memorial contributed $34,000 for the event, hosted by Seven television personality Melissa Doyle.