UPDATE 1-Australia to buy eight Boeing Poseidon P-8A spy planes

* First P-8As due in 2017, all eight fully operational by 2021

* Australia also has an option for a further four aircraft

* Australia has shown interest in MQ-4C Triton maritime superdrones

(Adds details of the purchase)

By James Regan

SYDNEY, Feb 21 (Reuters) - Australia will buy eight of Boeing Co's BA.N P-8A Poseidon long-range spy planes to help protect its offshore oil and gas interests and maritime borders, at a cost of about A$4 billion ($3.6 billion) including support facilities.

Australia is counting on the surveillance aircraft to dramatically boost its ability to patrol 2.5 million square kilometres (965,255 square miles) of marine jurisdiction -- an area equating to nearly 4 percent of the world's oceans.

The Indian Ocean on Australia's west coast has become one of the world's most vital routes for energy and raw material supply, with over 80 percent of China's oil imports transiting through the area. Japan, India and South Korea are also dependent on Indian Ocean routes.

China's navy this month conducted its first substantial military exercise in the eastern part of the Indian Ocean and in Australia's maritime gateways, sparking speculation it was showing greater interest in protecting its commercial sea lanes into the Indian Ocean.

"The P-8A is a potent and highly versatile aircraft," Prime Minister Tony Abbott and Defence Minister David Johnston said in a joint statement."

"As well as patrolling Australia's maritime approaches it can conduct search and rescue, anti-submarine and maritime strike missions using torpedoes and Harpoon missiles," the statement said.

The first P-8As will be delivered to the Royal Australian Air Force in 2017, with all eight aircraft fully operational by 2021. Australia also has an option for a further four aircraft.

Along with a fleet of high-altitude drones, the P8As are designed to replace the Royal Australian Air Force's AP-3C Orions, which have been in service for more than 40 years.

Australia has also shown interest in acquiring a fleet of MQ-4C Triton maritime superdrones at a cost of around $3 billion.

The Triton, under development by Northrop Grumman NOC.N , is the size of a small airliner with a 40-metre wingspan. It can cruise at 20,000 metres (65,600 feet) for up to 30 hours, sweeping a distance greater than Sydney to London with 360-degree radar and sensors including infra-red and optical cameras.

This would also provide surveillance compatibility with the U.S. Navy, which plans to buy 68 Tritons when they enter service in late 2017.

The P-8A is based on Boeing Co's 737-800 fuselage, structurally modified to include a bomb bay, under wing and under fuselage for weapons. ($1 = 1.1134 Australian dollars)

(Editing by Edwina Gibbs)

((jim.regan@thomsonreuters.com)(+612 9373-1814)(Reuters Messaging: jim.regan.reuters.com@reuters.net))

Keywords: AUSTRALIA MILITARY PLANES/