Three children playing in bush around Wellington's Ngaio Gorge have helped to solve the mystery of an engineer who disappeared almost 30 years ago.

The schoolboys stumbled on a skull and dentures concealed in the undergrowth in February this year, and ran home to tell their mother. She contacted police, who also found a watch, wallet, spectacles, some coins and what appeared to be the remnants of a hat preserved in the soil.

KEVIN STENT/FAIRFAX NZ The top of the Piwakawaka Track, around the area where John Holmes Smith was thought to be known to like to walk before he went missing for almost 30 years. His body was found in undergrowth lower down in the area in February 2015.

Police announced on Monday that they had now identified the remains as being those of John Holmes Smith, a pioneering geothermal engineer, who was 76 when he went missing from his home in Ngaio, northern Wellington, in March 1986.

He left his home in Kenya St about 2pm on March 20, wearing glasses, dark trousers, a brown jersey, and a checked fedora hat, to go for one of his daily short walks through bush around Ngaio Gorge.

He was never seen again, and his family had since "endured nearly 30 years of uncertainty", Wellington area commander Detective Inspector Chris Bensemann said.

SUPPLIED/NZ POLICE Spectacles found in the Ngaio Gorge area undergrowth with the body of Wellington retired engineer John Holmes Smith who was missing for almost 30 years.

That was until the three schoolboys ventured into a gully off Captain Edward Daniell Drive on February 16.

Forensic experts confirmed the remains belonged to Smith, who was married with a son and two daughters. Police said the examination uncovered no signs of trauma or foul play.

His son, Stafford Smith, said the family were sad to know he died alone, but grateful that they could now hold a private cremation and service for their father, after so many years of wondering what had befallen him.

SUPPLIED/NZ POLICE Wallet remnants found near Smith's body.

"Finding Dad's remains bring closure for our family. Dad loved walking and regularly walked in bush around the Ngaio area.

"While Dad was physically healthy, he did suffer from some mental confusion and we feared he had fallen or lost his way in the bush.

"Dad was a highly intelligent engineer and a well-respected member of the community. He was a much-loved husband, father and grandfather, and it's sad he died alone. We are pleased we can now lay him to rest."

SUPPLIED/NZ POLICE Coins found near Smith's body.

John Smith's widow, Nan, who died recently, told a hearing at the time that he came home one day, about six weeks before his disappearance, saying he had slipped and fallen but could not remember where or how.

His disappearance prompted a massive aerial and ground search, with 71 police and volunteers scouring the Ngaio and Wilton Bush area.

The searchers combed the Kaiwharawhara stream bed, as well as factories and warehouses at the foot of the Ngaio Gorge, working on the theory that Smith may have wandered into a factory or yard and become locked in over the weekend. But after four days and no evidence, the search was called off.

In May 1986, a memorial service was held for the retired former chief geothermal engineer for what was then the Ministry of Works and Development.

By August, with still no clue as to his whereabouts, the case was referred to the coroner, who found he must have died on or about the day he went missing.

Colleagues told The Evening Post at the time of his disappearance that Smith was a pioneer, who conducted one of the world's first investigations into how geothermal steam could be used as a natural energy source.

His legacy lives on in the Wairakei geothermal field near Taupo, now known as Te Mihi, which was the first wet-steam station in the world when it was commissioned in 1958. Today it has enough capacity to power about 160,000 homes.