The funds come on top of an estimated $139 million the Corkhill Brothers group stands to make from land sales under the deal reached with the former Land Development Agency by the Corkhills' subsidiary company Riverview Projects (ACT) Pty Ltd in 2015.

But the full cost to taxpayers of the 99-year contract for the work, the price calculation methodology and a second five-year contract for another Corkhill subsidiary, Riverview Sales and Marketing Pty Ltd, to act as real estate agent to market and sell the 11,000-odd residential blocks, remains confidential.

In a statement attributed to Riverview Group director David Maxwell and cleared by the government, he said the total value of the contracts was not defined in absolute dollar terms.

"Rather, it is dependent on the project’s revenues which in turn relies on how much of the land is ultimately developed and sold for residential purposes, at what price," he said.

Despite that, payments recorded in the government's notifiable invoices register show the company has been paid at least $12.1 million to date in "consultant project management" fees since 2015, a conservative estimate that would not include payments below $25,000 or those made before July 2015.

But when the former LDA signed the two contracts with the companies in 2016, it also exempted the firms from meeting the government's tender threshold requirements.

While such exemptions are allowed under the government procurement regulations, a formal reason must be provided, such as that it is an urgent procurement, a supplier has specialist or commercial information related to the work, or it is for unique equipment available from only one company.

But on both the contracts, the former agency provided no formal reason for exempting the contracts from the requirements, indicating the contracts may not comply with the government's own procurement rules.

While Riverview, as part of the deal, provided the land for the joint venture property development, it was not the only supplier in the ACT that could have supplied project management or real estate agents services, and the deal is unusual for the territory government which usually buys rural land to develop itself.

But Mr Maxwell said under the terms of the deal reached in 2015, the government agreed to appoint Riverview to the two key roles, a decision made because Riverview "developed the original intellectual property for the project, and undertook early vision, planning and design that was required to establish the viability of the project".

"It is not uncommon for a related entity to one of the joint venture participants to undertake the provision of project management as well as sales and marketing services for both participants under an exclusive arrangement," he said.

Neither he, nor the government, would say whether or not the company requested the exemption from the tender requirements, but he said that both parties contractually agreed to the exemption.

However, asked why Riverview requested confidentiality over the prices of the two contracts and price calculation methodology, Mr Maxwell's statement said such practices were common business practice in a competitive market, and a request for such information was always deemed to be commercial in confidence.

The two contracts cover Riverview's employment of 20 staff, including Mr Maxwell, which are employed variously on the construction and sales and marketing sides of the firm as well as some administrative roles.

Mr Maxwell said the project management fees paid to Riverview also help cover the cost of contractors and consultants the company enlists from time to time.

While the joint venture was only officially entered into in 2015, the statement also said the project had been subject to rigorous government review from 2006 and 2015.

The government has not directly answered the question as to why a formal reason was not provided for exempting both contracts from the usual tender requirements, as required under its procurement guidelines.