The Air Force has found replacements for its ageing 727 transport planes, but the bill could top $200 million.
The government says it will buy two 10-year-old Boeing 757 aircraft from Dutch carrier Transavia.
The 727s currently in use are 30 years old and have been plagued by mechanical problems.
Defence Minister Mark Burton says the cost is within the $100-200 million figure already set aside, but the full price will not be revealed till the planes undergo modifications.
They are having a new cargo door fitted in the main cabin and strengthened floors.
Burton says the new planes will go into service in April and June, and will be used for both military and civilian tasks such as emergency medical evacuations, disaster relief, military exercises, and VIP transport.
Transavia's website says it is phasing out the 757s as it modernises its Boeing fleet.