It follows concerns from former All Black-turned-firefighter Steve Devine who said at least one truck breaks down every day.
Devine challenged David Seymour to meet him at any station in the country so he could highlight issues each station's crews were facing due to underfunding.
Fire and Emergency NZ has rejected Devine's criticisms, with Seymour saying he had spoken to FENZ and they would "dispute some claims made".
On Tuesday, Seymour told RNZ he had spoken with Devine and could hear in his voice that he was very frustrated.
Seymour said he would not go "tit-for-tat" on every single issue raised, saying it did not help to go through them all.
"What is clear is that there does need to be a lot of upgrades and at the same time, it's only in the last two years that FENZ has had an asset maintenance plan."
Seymour said about $25 million each year was put into upgrades.
There was an "element of truth" in Devine's comments, but the trucks were not old as he had claimed.
Seymour said the trucks had been purchased new and were having upgrades made to them.
"You can't just get a fire truck off Trade Me, you have to actually really put the equipment on top and that involves some building."
But Seymour said he would continue working with firefighters on their concerns.
"Firefighters are just about the only group whose job it is to go into dangerous places ... firefighters are actually required to go into places precisely because they are dangerous so no one else will.
"I've long had that view when I've visited stations and talked to firefighters over the years. But they do do something that's out of the ordinary and I can understand exactly why Steve would have the concerns he does based on some of the things he said."
In a statement to RNZ, FENZ said it has been investing in replacing its fleet, with 317 trucks replaced since 2017 and another 78 in the pipeline.
"All the trucks are new, they have never been driven and are not old trucks that have been refurbished. 28 of the 78 are using chassis that are approximately five years old, but those chassis have not been used and were sent offshore for the new appliances to be built."
The remaining 50 appliances are being built on new chassis fresh from the factory.
"With a fleet or around 1300 trucks, faults are expected especially as they age, and they are reported most days, but that doesn't mean to say trucks "break down every day." We have service providers right across the country that maintain our fleet to a high standard through a robust servicing programme and they ensure our appliances are repaired and compliant when they do break down outside normal servicing."
