There are concerns at the highest levels of the Australian Defence Force (ADF) about the lease of the Port of Darwin to a Chinese-owned company.

The Northern Territory Government has signed a $506 million deal with Landbridge Group to lease the port and facilities of East Arm Wharf, including the Darwin Marine Supply Base, and Fort Hill for 99 years.

Under the agreement, hailed by NT Chief Minister Adam Giles as "a fantastic outcome for the Territory", Landbridge will take an 80 per cent ownership stake, leaving the other 20 per cent in Australian hands.

The ABC has spoken to a senior ADF official who said there were concerns about the security implications of a Chinese company owning a mounting port for major operations.

The Navy website said Darwin "is a vitally important Navy port ... and the centre from which we conduct border integrity operations".

Sorry, this video has expired Overview of proposed development at Port of Darwin

"Darwin plays host to major Navy and multi-national exercises and operations involving around 100 visiting Australian and foreign major warships each year," it said.

The deal comes as Australia and the United States agree to "enhanced naval cooperation across all domains" as territorial tensions simmer just north of Darwin in the South China Sea.

The company is owned by Chinese billionaire Ye Cheng and operates a large port in Shandong province in China.

It has interests in port logistics, petrochemicals, international trading and real estate development and, according to the AusTrade website, employs more than 7,000 people.

The deal has been condemned by the NT Opposition as "nothing more than another one-off cash grab", while unions claim it has been "shrouded in secrecy".

Landbridge said it was making a "considerable financial investment" in the port, promising to spend $35 million within five years.

"Landbridge intends to grow two-way trade between Australia and Asia, leveraging Landbridge's existing port and logistics businesses and firmly putting Darwin on the map for Chinese business," Landbridge's Mike Hughes said.