In this episode of Eyewitness Sonia Yee takes a look at why The Great Crunchie Train Robbery was a runaway success at a time when the advertising industry hit a turning point.

What do an old train carriage, a chocolate bar, a guitar and a lawn mower have in common?

The answer: They were all involved in the making of one of New Zealand's most iconic and longest running ads, The Great Crunchie Train Robbery.

Listen to: New Zealand's most iconic ad - The Great Crunchie Train Robbery

The commercial was made in 1975, and according to its director there were a number of components that contributed not only to its longevity, but also its success.

"I think it was the storyline ... the way that it develops and builds and goes into the surreal," says Tony Williams from his home in Kangaroo Valley, New South Wales.

Set in the Wild West, the story was bursting at the seams with distinctive characters, interspersed with black-and-white, World War II footage and scenes from old westerns that Williams had sourced through an industry friend working at Paramount Studio in Hollywood at the time.

"I got it for next to nothing," he laughs.

The story was also action-packed and filled with the kind of suspense that kept its audience hanging on.

As for the location, it couldn't be further from the Wild West, but was filmed inside an old train carriage in Lower Hutt, just outside of Wellington.

"We put tracing paper on the windows and had lighting outside," says the veteran filmmaker whose job it was to keep the action moving.

But the hardest task fell into the hands of the helpers and assistants outside the carriage who had to bring it to life.

"Every time we were about to do a take we'd say, 'rock the train,' and these poor guys were out in the middle of the night in the freezing cold, rocking the train with their crowbars."

Williams recalls a fun, party-like atmosphere on the set with a cast that included young actors Bruno Lawrence and Stuart Devenie, who were in the early stages of their careers.

"You couldn't get the guns off the guys during the morning tea breaks ... they were in their characters of cowboys and were practicing their fast draws," laughs Williams.

The commercial ran in New Zealand for more than two decades and was also a runaway hit overseas, winning lots of awards.

But when the director moved to Australia in 1980, industry insiders struggled to believe that this very successful ad had been made in New Zealand.

Williams remembers visiting an ad agency and handing over his showreel. The Great Crunchie Train Robbery was right at the top of his list…

Go to this episode on rnz.co.nz for more details