Dr James Bennett is the grandson of Francis Oswald Bennett, the Christchurch GP who was a key defence witness in the court trial after treating Pauline Parker, 16, who murdered her mother.
Parker and Juliet Hulme, 15, – suspected to be lesbians – lured Honora Parker to Victoria Park in the Port Hills in June 1954, where they bludgeoned her with a half-brick wrapped in a stocking.
Bennett's fascination with the case began when his aunt told him about his grandfather's connection, just after the movie's release in 1994.
As a gay man, Bennett said it was "perturbing" to see his granddad portrayed on screen as someone who found homosexual behaviour insidious.
In his report at the trial, Dr Bennett described the murder as "bestial and treacherous and filthy. It is outside all the kindly limits of sanity. It is 1000 miles away from sanity ... they are still not sane and in my opinion they never will be sane."
Bennett was moved to study the movie in the context of New Zealand's attitudes to sexuality in the 1950s and he has published a report in a history journal.
"The case caused a sensation in this country and overseas due to the barbaric nature of the crime, the relative rarity of a documented case of matricide, and because of speculation over the girls' sexuality," said Bennett, an Auckland University lecturer.
He remembers his grandfather as being almost totally immersed in his profession and hobbies, with little time for grandchildren.
"They lived in Mount Pleasant most of the time. I saw a reasonable amount of my grandmother but he would normally be buried in his workshop or working when I was there.
"I think the longest conversation I ever had with him was about 20 seconds. I was doing something on my bike and he had just driven into his garage and wondered whether I had had a puncture or something along those lines."
Although a published writer, Dr Bennett never wrote papers about the Parker-Hulme case. He died in the mid-1970s.
Half a century after the murder, Bennett had to rely on court transcripts, personal letters and family conversations for insights.
In a letter to his wife, Dr Bennett wrote of his pre-trial jitters. "I believe there are a number of foreign correspondents coming. The publicity glare will be fierce. The local glare doesn't matter, it's the historical one I fear."
Bennett said the movie portrayed his granddad as a caricature of the medical profession in the 1950s.
"One of the dramatic techniques the film-maker is using there is that he wants to deliberately exaggerate his characters to get across his message that the medical profession at the time connected mind and body, and medicalised sexuality."
Parker had been referred to the doctor by Hulme's father, the rector of Canterbury University, who feared the friendship between the girls had become "unwholesome".
Parker's main reason for the doctor's visit was to check on her significant weight loss but the movie hones in on Dr Bennett's mouth as he struggles to say the word "homosexuality".
Individuals who fell outside sexual norms were labelled deviates in post-war 1950s Christchurch, Bennett said.
The girls were seen "almost as some kind of virus which had contaminated Christchurch".
Dr Bennett and a psychiatrist noted an extreme conceitedness and arrogance in the girls, linked to a delusional world of grandeur.
Overall, Bennett thought Jackson portrayed the case well within the limits of film, and made the story accessible to a wide audience.
For authenticity's sake, the film-makers even went to the lengths of tracking down the Armagh Street house where Dr Bennett had his surgery.
The house happened to be still in the Bennett family at that stage and permission was granted for the crew to film the doctor scene there.
Both Parker and Hulme were found guilty of murder, but were too young to hang. They were sent to separate jails for almost six years.
They were given new identities on release, after which both left New Zealand.
Hulme, revealed in 1994 to be best-selling British crime novelist Anne Perry, has denied there was any sexual relationship with Parker.