Upgrading the Air Force's six Orions is expected to cost $330 million, which is less than originally estimated.
Responding to an Official Information Act request from Radio New Zealand, the Ministry of Defence has revealed the total cost of the seven-year project is projected to be $332.3 million.
The P-3K Orion Upgrade Project would finish halfway through this year with only a software upgrade to go.
The ministry's acquisition division deputy secretary, Des Ashton, said the original budget for the project was $377 million.
Exchange rates had played a role in why the project would come in under budget, he said.
The project was based in Texas and Blenheim with a mixture of overseas and local contractors.
Equipment such as computers, mission systems, radar systems, wiring and antennas had been upgraded.
"If you go back to the configuration of the aeroplane it was quite modern compared to some P3s in the world, but the computing power that was put in the original upgrade back in the 80s/90s was about the power of a commodore 64."
The Orions are used in search and rescue missions, fishery protection, anti-piracy operations and surveillance.
Mr Ashton said it was important that these planes were upgraded since the world had changed since the planes were first introduced into service in 1965.