Australia on Friday said it will buy eight Poseidon aircraft for Aus$4 billion (US$3.6 billion), which will form the core of its surveillance andmaritime strike capacity for decades to come. The Boeing planes will be used to monitor over 2.5 million square kilometres of the country's marine jurisdiction -- some four percent of the world’s oceans. "The first duty of government is the defence of the nation. This is a government which is absolutely committed to the border security of our country," said Prime Minister Tony Abbott. "This fine and highly capable aircraft will be at the heart of oursurveillance and maritime strike capacity for the decades to come." Australia, a close ally of the United States, is expected to use the planes to patrol far over the Indian Ocean, which has become one of the world's most vital energy supply routes. The new aircraft will replace Australia's ageing Lockheed AP-3C Orion aircraft. The P-8A is described as highly versatile and able to "conduct search and rescue, anti-submarine and maritime strike missions using torpedoes and Harpoon missiles". Chief of the Air Force, Air Marshall Geoff Brown, said they would give Australia a cutting edge advantage in protecting its borders. "The P-8 gives us an unprecedented capability to find, fix and track both surface ships and submarines, in combination with our wedged tail aircraft which have an unprecedented ability to attract airborne targets," he said. "It will certainly help the Royal Australian Navy be able to have freedom of manoeuvre right through our region for the next 20 or 30 years." Abbott played down their possible use in halting the flow of asylum-seeker boats that depart from Indonesia and Sri Lanka. "We expect the first one to be in operational service by 2017 and I think the boats will be stopped by then," he said. Halting the flood of asylum-seeker boats arriving in Australia's north has been a key policy of the conservative government since it was elected last year. Reports recently said Australia also planned to purchase seven giant unmanned US-made Triton drones for Aus$3 billion for border patrol duties. The use of large unmanned aircraft has been mulled for a decade in Australia but the previous Labor government would not sign off on the concept as it reportedly believed the technology was not mature enough.