"He has deteriorated quite badly over the years," he said.
The stained-glass face of Jesus, painstakingly cut by Mr Mackenzie from the south-facing stained-glass window at Knox Church is just one of many faces and hands which have discoloured over the years.
The large window also features the 16th-century Scottish reformer, John Knox, after whom the church was named; St Andrew, St Margaret and Dr Donald Stuart, the first minister of Knox Church.
Only Jesus and the evangelists require work.
The glass painting had faded because it had not been fired at the correct temperature, Mr Mackenzie said.
"You can tell that by the way the paint is just falling off in places."
By not being fired at 650degC during a series of paint firings it was likely the glass had started deteriorating within 10 years of being installed.
The window was made by renowned artist Robert H.Fraser, of Dunedin, in 1896, "a master of his craft", Mr Mackenzie said yesterday.
It was a privilege to work on such fine examples of stained-glass windows, Mr Mackenzie said.
He is one of only a handful of master glass painters in the country.
"There is no better place in the country than Dunedin to do this job. I enjoy exacting restoration. It doesn't feel like work."
Using surgical steel instruments normally seen in a dental surgery, Mr Mackenzie carefully cut the faded glass from its lead surrounds.
The glass was painstakingly removed and cased for transport, to be repainted, fired and reinstalled, after hundreds of hours of studio work.
The restoration is part of a $100,000 upgrade of Knox Church, which includes the installation of a sprinkler system, cleaning and revarnishing of the ceiling and general maintenance on the organ.
