When journalist and author Lech Blaine was 11 years old, his mother Lenore would often joke she could write a book about the trouble that lay just beyond their Toowoomba driveway.

At the time, Blaine couldn’t imagine anyone wanting to read about Michael and Mary Shelley, and the “visceral terror” these two outsiders inspired in him and his siblings. Mary, with her purple dress and slightly posh accent, appearing on their doorstep calling his mother “Satan’s handmaiden”, calling his siblings by unfamiliar biblical-sounding names like “Saul” and “Joshua”. Michael, sitting ominously in the White Chrysler outside, already a convicted kidnapper with a long and colourful rap sheet. Blaine’s family would grow familiar with the wild, all-caps letters that would appear in their letterbox, and the police cars that would follow a visit from the two strangers who wrote them.

“I’d been so terrified of them, even when I was still a teenager,” Blaine explains, now 32. “Even after I’d moved away from Toowoomba I used to dread their potential arrival.”

It could seem there was no escaping the Shelleys. In his latest book Australian Gospel: A Family Saga, Blaine tries to make sense of the deep ties binding the families together.

Blaine’s siblings Steven, John and Hannah had been born Saul, Joshua and Hannah Shelley – the biological children of Mary and Michael. But they were, separately, removed as babies and toddlers from their care by social service workers concerned about their treatment and placed into the care of foster parents – Tom and Lenore Blaine. Mary and Michael would never stop trying to recover their children; by law or by threat of force.

The Shelleys were a pair of self-styled Christian prophets sharing their custom blend of Old Testament brimstone and back-to-the-earth hippie culture with anyone who listened.

For years the pair had hitchhiked their way around Australia and New Zealand, leaving a scorched-earth paper trail across courtrooms, gaols and newspaper columns. They quickly burnt through the goodwill of anyone who helped them, and waged scornful campaigns of harassment against those who didn’t.

That placed them on a collision course with the Blaine family; two working-class parents and their chaotic brood of rugby-loving foster kids with matching back yard haircuts, being raised against the backdrop of small-town country pubs. To the Shelleys, they represented everything that was morally and spiritually corrupt about modern Australia.

For years, their children’s new identities, foster family and location were a closely guarded secret. Finding them, and recovering them, became the Shelley’s obsession. They spent decades harassing social workers, sending death threats to the premier of Queensland, and in 1983 kidnapped their eldest son, Elijah, from his foster home.

Despite the restraining orders and stalking charges, the Shelleys would haunt the Blaines for years, with a near-constant stream of threatening and pleading letters sent from wherever Mary and Michael were in the world.

“The contempt I feel for you two child abusing deviates is profound and deserved,” Michael would write in one email to Lenore, adding, “I rejoice in where you are both going – HELL!”

These tirades formed part of a long and knotty paper trail that Blaine would base his book on.

Blaine began piecing together the story after moving back home at 21. His mother had been diagnosed with a rare and terminal neurodegenerative illness, and as he tried to make sense of her future, he also found himself grappling with the family’s past.

“She’d kept this meticulous record of everything and passed all that stuff on to me,” he explains. “So I spent that summer organising her nursing home placement, selling the house, and going through basically everything that she had.”

There were years’ worth of diary entries, newspaper clippings, social service reports and, more recently, a decade of emails that the Shelleys had inundated her inbox with.

“I got so addicted to information at times,” he says.

“At that point, thanks to a lot of the information that Mum had kept, I realised how much more interesting the Shelleys were than these really quite terrifying, monstrous people in my imagination as a child.”

With his mother too sick to write the story, Blaine resolved to do it himself.

His mother’s archive told one side of the Shelley story. But as he began to reach out to social workers and other witnesses, Blaine knew there was another source he needed to hear from: Michael Shelley.

“I lived in absolute fear of him,” Blaine says. Nevertheless, he sent him an email. “I actually still can’t believe that I really did it.”

Michael responded to Blaine’s first tentative email and was soon sharing his own personal archive of over 400,000 words of material including unpublished autobiographical accounts, reports and sermons. Even from someone Blaine knew was an “incredibly unreliable narrator”, it created a vivid picture.

Blaine’s siblings were burnt-out from years of Michael’s fiery attempts to reconnect – often by accusing his children of being a “TRAITOR”, “brainwashed” by authorities and the Blaines. But Lech Blaine’s correspondence struck a different tone to the harassing messages his family had received for years.

“It was pretty civil,” Blaine recalls. “Occasionally he would go on some rants, but he never really got vicious with me. I think that he was more angry at my foster siblings because they weren’t paying him any attention or trying to get in contact.

“This is a guy who had spent decades desperately trying to get people to read his writing and to ask him what he thinks about things. I was really one of the only people who’ve ever actually showed much interest in what he had to say.”

Shelley’s own voluminous writings filled in the gaps in the public record, his mother’s records and Blaine’s own childhood memories.

“I got a much better sense of who they were before they’d suffered nervous breakdowns, and I got a genuine sense that they weren’t evil. They weren’t irredeemably awful. From their birth they were rich, complex people who had quite serious, especially in Mary’s case, quite serious mental health issues.”

In a previous life, the Shelleys had been charismatic, privileged Sydney socialites whose relationships and exploits had garnered magazine front pages and newspaper column inches.

The Shelleys found each other in the wake of breakups and breakdowns, beginning a decades-long co-dependency that saw them drop out of mainstream Australia for good, no matter the cost.

As the book took shape, Blaine was also committed to recognising how his own parents’ complexities shaped their family experience. He could see how his “larrikin” Dad’s sense of humour was a “coping mechanism for some of the things that he suffered when he was quite young”. He understood how his mother was an excellent foster carer because she was nonjudgemental, “she didn’t radiate any sense of superiority to children”.

Lenore and Tom Blaine, and Michael and Mary Shelley all passed away years ago and as Lech Blaine worked on the book, his siblings wanted the same treatment in the book as their elders: to be seen as complex, not caricature.

“They weren’t expecting me to paint like a rose-coloured portrait of them,” he says.

Blaine did not want, either, to paint a rose-coloured portrait of hope in modern Australia. Through tracking the lives of his siblings and their siblings, Blaine shows that whether someone’s life becomes an Australian dream or nightmare can hinge on an opaque mix of nature, nurture, systematic factors beyond most people’s control and sheer luck.

The final result, Australian Gospel, is a big-hearted epic, where the pangs of terror are never far from the next belly laugh.

To understand the story from his siblings’ perspective, Blaine called them every few nights, talking for over an hour at a time. “That went on for years,” he says. “I think it just created a real intimacy.”

As children, the Shelleys had threatened to tear the Blaine family apart. As adults, piecing together their story helped bring them even closer.

• Australian Gospel: A Family Saga by Lech Blaine is out on 5 November through Black Inc Books