The Defence Force is spending $59 million on new LMT MARS-L rifles, replacing weapons which had been ineffective at certain distances.
Defence Minister Gerry Brownlee said Cabinet had approved the purchase of more than 9000 weapons, which were the preferred choice of Defence Force personnel.
The price tag includes additional equipment and training.
The rifles are made by Lewis Machine and Tools in the United States.
They work with night sights, grenade launchers, laser sights and sound suppressors.
The new light modular assault rifles will be used by all Defence Force personnel, including the special forces.
Mr Brownlee said they are "combat-proven" in arduous conditions.
"It gives our personnel a weapon they can rely on in any situation."
The Defence Force has had issues with the current Steyr 5.56mm rifle it has used for the past 28 years.
The Steyr was found to be ineffective at sighting targets and firing at ranges greater than 200m.
An upgrade was approved in May 2009, but a 2011 report found delays in the roll-out.
The Defence Force considered eight new rifle options before choosing the LMT MARS-L.
The first rifles will be in service next year.