Michael McCormack believes people will eventually congratulate the federal government for paying 10 times more than the land was worth.

"That eventually will be hailed as a good decision," he told 2GB radio on Monday.

"I appreciate that yes, it was very much over the odds, I appreciate there's a review going on into how that actually happened.

"But eventually when there is a need to be more runways and more infrastructure built at Western Sydney Airport, they'll look back and say, probably, what a bargain that was."

Mr McCormack, who is also the Nationals leader and infrastructure minister, admits there should have been better processes around the land purchase.

"But there has been a decision taken," he said.

"The $30 million, yes it's a lot of money, but in time it will be a very good investment."

Labor frontbencher Catherine King was quick to pounce on his remarks.

"For the deputy PM to tick off these processes is simply extraordinary and calls into serious question his judgement when it comes to the billions of dollars of infrastructure spending he oversees," she said.

"Today's comments again make it clear that the Morrison government simply cannot be trusted when it comes to infrastructure spending."

The Australian National Audit Office handed down a scathing report into the land purchase last week, finding Department of Infrastructure officials had engaged in unethical conduct and failed to ensure proper probity.

The department has since appointed an independent investigator to examine the land deal and staff conduct throughout the purchase.

The auditor-general found the department did not show appropriate due diligence in buying the 12 hectare Leppington Triangle.

It paid 22 times more per hectare than the NSW government spent on its portion of the land.

The department has agreed to improve the way it gets valuations, prepare more detailed cost-benefit analyses, and run proper meetings with landholders.

Australian Associated Press