The top official for vetting foreign investment was so concerned the Port of Darwin would be leased to a Chinese company that he asked three times for the Department of Defence and intelligence agencies to consider the national security implications "at the highest possible level".

The Foreign Investment Review Board (FIRB) and the Department of Defence are under scrutiny over the Northern Territory's decision to lease the port to Landbridge, which has links to the People's Liberation Army, and was allowed to take place through a legal loophole that made it exempt from federal approval.

The board's chairman, Brian Wilson, said he became aware of the deal in late 2014 and asked for a response from Defence and intelligence agencies in early 2015 and again in March. After a second response from Defence expressing no reservations to the deal, Mr Wilson asked for it to be considered a third time "at the highest possible level".

Former Director General of ASIO David Irvine at the AFR Defence and National Security roundtable in Sydney. Jeremy Piper

Had he known the Australian Navy would have guaranteed access to the port for only the first 25 years of Landbridge's 99-year lease: "I would have asked the question of the Navy again for the fourth time, and I would have had to accept it."

"It is truly on their heads; they are the experts here," he said.