New Zealand has signed a $1.6 billion deal with the US government to buy five navy helicopters and weapons.

It is one of the biggest single purchases so far under the Defence Capability Plan.

The Seahawks have anti-submarine torpedoes and air-surface missiles.

The Defence Force has been looking into this for months and the US government agreed to it in June under its Foreign Military Sale system.

The NZDF says the Seahawks can fly faster and further than its old choppers, increasing the frigates' offensive, defensive and surveillance capabilities.

The choppers made by a subsidiary of Lockheed Martin are due to arrive from 2030 at Whenuapai air base.

They will cost an additional $110 million a year to operate, plus funding would be needed to upgrade the air base.

One reason Defence went with this deal is that Australia operates Seahawks.

Drones that fit with the choppers may be added later, NZDF said.