Australia will spend $7 billion on six unmanned spy planes that can stay in the air for more than a day and will be used to hunt enemy submarines and carry out surveillance flights.

Key points: Drones will be used to detect foreign naval ships, illegal fishing or people smuggling boats

Drones will be used to detect foreign naval ships, illegal fishing or people smuggling boats First of the six new remotely controlled planes will not be in operation for another five years

First of the six new remotely controlled planes will not be in operation for another five years Information gathered will be shared with US, UK, Canada and New Zealand

Prime Minister Malcolm Turnbull said the Government would be spending $1.4 billion on the first of the six remotely controlled planes.

But the first of the new Triton drones will not be in operation for another five years.

Once they are in the air, the South China Sea is expected to be a prime target for the long-range, high-tech planes.

Their long-range design means they will be able to sweep well into the Indian and Pacific Oceans to detect foreign naval ships, illegal fishing or people-smuggling boats.

They will also be able to reach as far south as the Antarctic.

"It is very important for us to know who is operating in our area and therefore be able to respond if necessary to any threats," Defence Industry Minister Christopher Pyne said.

He said the Tritons would continue Australia's capacity to monitor South-East Asia and the South China Sea.

"Australia insists on its right to be able to travel through the South China Sea in international waters as we have always done, whether that is with surface ships or with aircraft," Mr Pyne said.

US Navy to help with development of drones

Greens senator Peter Whish-Wilson has questioned the Defence Minister over what he believes is a more than 100 per cent cost blowout in the Triton program, since it was first announced in the Government's Defence White Paper in 2016.

"Only two years ago in its White Paper the Government said that they would be acquiring seven drones at the cost of between $3 and $4 billion," Senator Whish-Wilson told Parliament.

"But two years later, they're acquiring six of these Triton drones, so one less, for $7 billion.

"That's a more than a hundred per cent blowout in costs."

Defence Minister Marise Payne denied there has been a blowout.

She told PM the "total cost of the six aircraft, and the associated systems and the infrastructure and estate works that need to be done, is estimated at about $3 billion, which is absolutely within the provision of the integrated investment program".

Senator Payne said the $6.9 billion dollar figure includes whole of life sustainment costs for the planes.

The full fleet of six Tritons is due to be in operation by late 2025.

The information the planes gather will be shared with Australia's allies the United States, the United Kingdom, Canada and New Zealand.

They will all be based at the RAAF base in Edinburgh, South Australia, and they are designed to operate closely with the US military.

The Tritons do not carry weapons because they are designed purely for surveillance.

The 2016 Defence White Paper included a plan for seven Triton drones, but the Government's latest announcement is for only six.

Australia is buying the spy planes from the giant US military company Northrop Grumman.

The deal includes a $200 million joint program with the US Navy for the development, production and maintenance of the Tritons.