The Australian government is to spend more than $5 billion on long-range surveillance drones to beef up its maritime security in the South China Sea – and it’s costing Canberra more than double the original estimated price.

The multibillion dollar military investment was announced on Tuesday, with the government vowing to buy six MQ-4C Triton remotely piloted aircraft from the US Navy. The program will cost the Australians $5.1 billion (AU$6.9 billion), which includes the purchase of the drones and infrastructure investment.

The Northrop Grumman MQ-4C Triton has a wingspan equivalent to that of a Boeing 737, and can remain airborne fro more than 30 hours at a time, surveying almost 3 million square miles of sea in one flight.

Australia’s first Triton will take five years to come into service, while the full six-aircraft-strong fleet is to become operational in late 2025. The Tritons are to join the Australian Air Force’s existing fleet of P-8A Poseidon aircraft and jointly “undertake a range of intelligence, surveillance and reconnaissance tasks,” a government statement reads.

The move is aimed at enhancing Australia’s “anti-submarine warfare and maritime strike capability” as well as border protection and making the “region more secure.”

The investment into the drones has sparked a debate at the parliament, with member of the Liberal Party of Australia Kevin Andrews questioning whether it would enhance the capability of defending the country’s national interest.

Minister for Defense Industry Christopher Pyne stood by the investment, even as it is now costing the Australian taxpayer more than double the estimated AU$3-4 billion n 2016.   

“One of the most important things we do as a nation as part of the Five Eyes is the reconnaissance and surveillance of the Indian Ocean, the Pacific, South East Asia and of course to Antarctica,” Pyne told the MPs. Australia cooperates with four powers – Canada, New Zealand, the UK and the US – in the global intelligence alliance known as The Five Eyes.

Pyne told Sky News that the operational area of the drones will cover the South China Sea. He added that Canberra insists on its rights for free movement in the region – the claim Beijing has repeatedly rejected, as it considers the waters of the South China Sea its national territory.

“Australia’s responsible for about 10 percent of the world’s surface into the Indian Ocean, the Pacific, down to Antarctica up into the South China Sea,” Pyne said.

China’s claims over the region are challenged by several other regional players, including Vietnam, Brunei, Malaysia, and the Philippines. At the same time the US, which Australia views as its “most important defense” ally, insists on freedom of navigation in the area and often sends its warships to sail through the disputed waters, drawing ire from Beijing.

In addition to spying over the South China Sea, the drones will be used to monitor vessels in Australian waters, including other countries’ naval vessels, and keep an eye out for people smugglers and illegal fishing activity.