"During World War II there were lots of left-over parts here in New Zealand which meant a lot of people were coming here for parts. Through the favourable exchange rate, especially in the 90s, we were able to convince them to rebuild here too. Many of them have multiple aeroplanes.
"Those first few jobs established our reputation. It's a lot of word of mouth," says Mr Denholm who, like most of his staff, has a background in aircraft engineering.
He joined the Air Force from school, starting at the Woodbourne base in Blenheim, but lasted only a year because the job didn't suit him.
"I left and got a job looking after scenic flight planes. Then I came to Auckland from Rotorua and started working on what was the first warbird, the first major plane owned by an Auckland syndicate.
"It's a very small group who are mostly fascinated by World War II planes. Even today those planes provide performance that you can't get anywhere else," he says.
So how much would it cost to bring a warbird back to its former glory?
"Oh, we're very reasonable," says Mr Denholm, grinning before divulging that it's between US$1.5 million ($1.9 million) to US$3 million.
And the global recession doesn't have much of an impact on his clients.
"Everybody should have an oil baron on their books. It helps when your best clients are oil tycoons and oil is selling at more than $50 a barrel."
While planes find their way into the AvSpecs hanger from some odd places - like a Spitfire from an English museum that had its wings sawn off because it wouldn't fit the space - Mr Denholm says finding the parts and original plans for the planes can be a challenge, too.
"They can be hard to track down. We'll find them in old libraries and museums around the world. We had a guy in Thailand get in touch recently because he had an old engine and wanted to sell it."
But the work isn't limited to rich overseas warplane collectors. Celebrity chef Simon Gault had his 51 Thunder Mustang - a replica warplane - built at AvSpecs six years ago as well, with a $1m price tag.
Mr Denholm says the public are always interested in the projects undertaken at his business and the site gets plenty of visitors.
"We try to discourage guys from getting in and making engine noises and spitting all over the cockpit, though," he says.
But they always welcome families with a connection to someone who may have flown a particular plane, or the original pilots themselves.
"I try to get videos of them telling me what it was like. Some of them can be very quiet about it because they were affected by it. Others are more like petrolheads.
"But we do try to get the story of what it was like for these guys. Many of them were only 18 or 19 years old when they flew."
The restored planes are often taken to shows and have been used in films for the likes of Sir Peter Jackson, who is a vintage plane enthusiast. "But he's a World War I guy, whereas we tend to concentrate on World War II," Mr Denholm says.
He shows a photo of himself and Jackson in the director's Wellington hangar. "He has all these amazing old photos of guys with their handlebar moustaches, a cigar, a dog and their plane crashed in the field behind them."
Wrecks that might, one day, find their way to a workshop in Ardmore.
For more information see AvSpecs' website, www.warbirdrestoration.co.nz
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