Federal officials are set to face a grilling about a controversial land purchase near the site of a new airport in western Sydney.

The Australian Federal Police has opened an investigation into why the federal infrastructure department paid more than $30 million for the parcel of land when it was only worth $3 million.

Deputy Prime Minister Michael McCormack, who is responsible for infrastructure, still insists the purchase was a bargain for taxpayers.

Mr McCormack said the price tag would eventually make sense once new runways and other infrastructure were required at the site.

“The price paid now, well, at least they won’t be having to buy it and probably pay considerably more than $30 million for it then in decades’ time,” he told ABC radio on Monday.

Labor frontbencher Kristina Keneally plans to probe the “real estate rort” during Senate estimates hearings this week.