The planes, a modified version of the Boeing 737 commercial airliner, will replace an ageing fleet of six P-3 Orions which have been in service since the 1960s, Defence Minister Ron Mark said.

"The purchase enables New Zealand to continue to deploy in a wide range of airborne maritime situations independently and, when required, work with partners including Australia, the United Kingdom and the United States," he said.

New Zealand is responsible for a huge search and rescue area over the Southern Ocean and South Pacific. Wellington also regularly deploys military assets for regional humanitarian relief efforts.

Mark said the Poseidons had a greater speed and range than the aircraft they will replace and were essential for New Zealand's "ability to contribute to global security efforts".

Approximately half of the budget would go towards purchasing the planes, which have state-of-the art radar and high-definition cameras, while the rest would be spent on costs such as infrastructure and flight simulators, Mark said.

The new planes will begin operations in 2023.

The purchase comes just weeks after close neighbour Australia announced plans to invest $5.2 billion to develop and buy high-tech US drones for joint military operations and to monitor waters, including the South China Sea.

They will complement the seven P-8A Poseidon planes Australia currently uses.

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