Warning: Contains spoilers.

The hero of popular new Netflix miniseries 'The Serpent' says he found the show "absolutely gripping" - and "formidable" in its dedication to accuracy.

Former Dutch diplomat Herman Knippenberg, who is now living out his retirement in Wellington with his Kiwi wife Vanessa Knippenberg, has spoken about how the case of French serial killer Charles Sobhraj has continued to resurface in his life like "tropical malaria" nearly half a century later.

Knippenberg has found his involvement in the case thrust into the spotlight once more following the recent release of The Serpent, an eight-episode series detailing Sobhraj's trail of violence through Thailand, Nepal and India in the 70s, and Knippenberg's relentless efforts to bring him to justice.

"Even one of my classmates said this was not really diplomatic work. There, I disagree," he told the Herald.

"In essence, the Netherlands' interests are, in my terms, the interests of the population."

For Knippenberg, that meant when the worried parents of two young Dutch tourists asked for his help locating them, he was a dog with a bone.

He discovered the pair had been brutally murdered and robbed, but felt he had a duty to continue investigating their deaths.

"I'd seen people dead before but I had not seen them dead in that manner. To see this and to hear from the Thai medical specialist that these people had died, that when the gasoline went over them and was set alight they were still breathing, at that moment I said to myself 'it's not enough to send a cable to Holland that these people have died,'" he said.

"The priority is to stop the person. Even if it's not completely my task I will do what I can."

The show follows Knippenberg and his dedicated team as they work to connect Sobhraj to a cluster of tourist murders. He is thwarted at many turns by bureaucracy and failures from other authorities, and is actively discouraged from pursuing the case.

Watching the show left Knippenberg feeling as though the scenes were melding with his memory of events.

"It was absolutely gripping, particularly certain scenes," he said.

"It was done in such a realistic fashion that I had difficulties watching it to flash back to my own memory ... was it as I remembered it or was it as I saw it? It was that close."

"Over the board I would say it has a very high degree of accuracy but with the understanding of course that it is intensified."

For example, a scene where a "completely mentally broken" Knippenberg wades into a pond to tear lilies out and take them back to his home didn't actually happen, though he was told in reality he could get some staff to help take some of the lilies to replenish his own pond.

"I am not sure that I was close to a breakdown but let's put it this way: For the first, until now, 45 years later, only time in my life I was depressed," he said.

"I was depressed and I remember only in that period I considered resigning."

The frustrations kept coming as Sobhraj continued to find ways to evade capture, and the case continued drawing Knippenberg back in as the years went on, like "tropical malaria".

His favourite part of the show is the end, which reveals Sobhraj being captured in Nepal in 2003.

Knippenberg, who is living in New Zealand, by that point, receives a call that the arrest has been made, and is able to provide documents to the authorities to help prove Sobhraj's connection to the Nepal murders.

In a detail too far-fetched to be included in the show, Knippenberg revealed to the Herald that call came on his very first day of retirement, bringing his involvement in the case to a close just as his career as a diplomat came to an end.

Despite his important role in bringing about justice, Knippenberg said he was not a hero.

"I think it's a tragedy in the terms that here the main person is a case of a tragic misuse of a very, very gifted mind."

The Serpent is streaming now on Netflix.