Defence Minister Ron Mark is now under pressure to replace the Air Force's ageing Hercules fleet before their useby date in three to five years.
The Hercules fleet frequently fly on humanitarian aid missions but are more than 50-years-old.
National's defence spokesperson Mark Mitchell said the fleet was arguably more of a priority than the $2.3 billion upgrade of the P-3 Orions that was announced on Monday.
The Lockheed C-30 Hercules planes have already undergone a life extension programme, commissioned by the previous Labour government. It chose that maintenance instead of replacing them, at a quarter of the price.
But the programme took much longer and was far more costly than anticipated.
Senior fellow at the Centre for Strategic Studies Dr Peter Greener said that is typical defence decision-making of successive Labour and National governments.
"We like to wring as much life out of equipment as possible," he said.
"We've ended up with two sets of aircraft which are now over 50 years old. They're like the hammer with the new head and the handle, in that many parts have been replaced over time.
"There is now a finite life, for both the air frame and the systems within them."
The Defence Force spent around $360 million on maintenance and repairs on the Hercules and Orion planes over the past 10 years - twice as much as the decade prior.
The decision to replace the Orions before the Hercules was because there was a risk New Zealand would miss out on the same marine surveillance aircraft its defence partners use if it didn't put an order in now.
The US company Boeing is about to stop taking orders for that model as it moves to upgrade the production line.
Ron Mark told reporters on Monday the country would miss out on interoperability with our defence partners if he didn't act.
"Crews being able to change over between planes, our operators being able to operate the gear in the back. It doesn't matter if we have a Canadian or an American or an Australian - we'll all be interoperable in that."
National's defence spokesperson Mark Mitchell said he's pleased to see the Poseidon purchase signed off - but it's vital the Hercules get the go-ahead too.
"When you talk about climate change and weather-related issues in the Pacific, it's actually the Hercules that do the heavy lifting in terms of getting medical aid and supplies into remote or tough areas to access."
Mr Mitchell said he knows what decision he'd make if he were still the minister.
"You could make the argument that the Hercules are probably more important than the P-3 Orions. It was only because we had to react quickly to last orders on the Poseidons, but the Hercules were actually priority.
"So they're going to have to act fairly quickly."
Mr Mark said his preference was to wait on making a decision until the capability review was complete, but that's not due until November and he admitted work may have to start earlier than that.
That capability review is looking into all of the procurement plans of the previous government.
Mr Mark said he knew time wasn't on his side and there will be a point in the near future when the Air Force will no longer be able to run the Hercules safely.
The Air Force has been flying three of the C-130 Hercules since 1965 and two more since 1968.