One of at least seven women subjected to sexual violence by a cowboy figure of the remote Whanganui River tells her story on television on Sunday night.

The Monster of Mangatiti is the 70-minute saga of Heather Walsh's treatment by William "Bill" Cornelius in the Mangatiti Valley during five months in 1985.

The programme could be shocking, and police have warned Mr Cornelius' other known victims that it will be shown.

Cornelius faced 22 charges, including rape and abduction, in Whanganui District Court in June 2012. Fourteen of the charges related to Mrs Walsh's time in the Mangatiti Valley.

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Victims of sexual abuse always have their names suppressed, but Mrs Walsh asked to have her name suppression lifted because she wanted people to know what Cornelius was like.

She told a Chronicle court reporter at the time he had used his home against "so many young women".

"In that way the girls who ride their horses in the valley or go down there will be warned," she said.

She has spent years recovering from the 1985 ordeal but was now "in control of her life", according to the programme's executive producer Philly de Lacey.

Mrs Walsh is now working with another sexual abuse survivor, Louise Nicholas, as an advocate for victims.

Ms de Lacey was intrigued by the Cornelius case when she heard about it. She contacted Mrs Walsh, who agreed to tell her story of survival hoping it would help other people.

Heather was a teenager when she got the job of tutor to Cornelius' son.

She was taken to his hut in the Mangatiti Valley, leaving Murumuru Rd on a vehicle track through 12km of dense bush. From the road end it is at least another 40km to Raetihi.

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Cornelius was charming and treated her "incredibly well" at first.

"He was grooming her for later on," Ms de Lacey said.

Then came rapes, starvation, and threats of torture and death.

The young woman became pregnant, but miscarried. Finally she escaped by stealing a utility while Cornelius and a friend were out hunting.

The Monster of Mangatiti is narrated by her, and acted by others. It screens on Sunday at 8.30pm on TV1, as part of the Sunday Theatre season.

The programme was filmed from December 2014 to March this year, and used information from Mrs Walsh, and from police and court files. Some footage was shot in the Raetihi area, but most was filmed in rural south Auckland.

"We've spoken to a lot of people and we had researchers in Raetihi," Ms de Lacey said.

The film crew never went to Cornelius' house but they flew over and saw it from the air. They had also seen it on television.

His remote farming venture was profiled on Country Calendar in the early 1980s. The Holmes show was back later in the decade, when Cornelius was having a feud with neighbours.

The Monster of Mangatiti crew already knew some of the stories of the backblocks Ruatiti Valley. They had made a show about the disappearance of Lionel Russell from nearby Mangapurua Valley in 1975, after talking to Cornelius' nephew.

When she heard how Cornelius had kept women captive, Ms de Lacey wondered how such a thing could happen in New Zealand.

The isolation of the place was one answer.

"It allowed him to hold those women in a very vulnerable position. It was an environment that he controlled."

Another was the effect of psychological abuse.

"What really intrigued me about this case is that it explores the psychological breakdown of a victim - he was a master manipulator. You could ask why she didn't just leave, but it's not that simple.

"We've got a massive issue with domestic violence in New Zealand. The film, hopefully, goes some way to explain it from a victim's point of view."

It took four years from Mrs Walsh's approach to police in 2008 to get the charges brought against Cornelius. By that time he was 79, and suffering from early stage dementia.

Judge David Cameron decided he was mentally unfit for trial but that, on the balance of probabilities, there was enough evidence to find he committed the offences he was charged with.

He was allowed to return to his remote valley, under supervision, and died there a few months later.