He originally had this idea, for book number four, about a story set in a huge coal mine, or an iron ore mine, somewhere isolated, where fly-in/fly-out workers complicated things even more, somewhere in South Australia, perhaps the Flinders Ranges.

He'd never been out that way and was planning a road trip. But then COVID struck and all the borders closed. Half the country was out of the question.

And then the inspiration hit much closer to home.

"I was out shopping in (Canberra shopping centre) Manuka and I caught a glimpse of a woman wearing this large opal pendant and I went, oh, an opal mine, that would be good," he said.

So he got in the car and headed to Lightning Ridge, about half an hour south of the Queensland border, and squeezed in some research before things shut down even further.

"I met some really interesting people, some opal miners were happy to take me down their mines, talk to me about the lifestyle in these places, what went on.

"While the town Finnegan's Gap is loosely based on Lightning Ridge, it's smaller and wilder, I'm not sure too many tourists would be pulling up there."

The tagline of Treasure and Dirt is "In Finnegan's Gap, opals, bodies and secrets don't stay buried forever". You're not going to find that on a local council tourism sticker.

In the opening chapters the body of an opal miner is discovered; he's been crucified and left to rot in his mine.

Before too long, the investigation unearths more bodies, the local religious fanatics are involved, so too corruption as mining magnates take an interest in the area. By the end, it has turned into that big story Hammer was thinking of in the first place.

Setting seems to define what's now known as "Australian noir", whether it's the outback (Jane Harper's The Lost Man is the best here for me), rural towns (try Sarah Bailey's The Dark Lake), small coastal villages (Peter Temple's The Broken Shore) or indeed the dark streets of the inner city (Mark Brandi's The Rip).

Sure, every story, whether it's been written in Dubbo or Dublin, needs a setting, but Australian authors have a knack of making that quintessential Australian place a crucial part of the story.

But at the end of the day, Hammer agrees that a good crime novel is defined by motive. There's a theory that it always comes down to one of the four Ls: lust, love, loathing or loot.

"You just can't get to the end of the book and tell the reader someone was just having a bit of an off day so they killed someone," Hammer says.

"But the setting can help explain those motives. In Scrublands, an irrigation town has run out of water and people are desperate and desperate people do desperate things; in Silver, set on the coast, it's motivated by real estate greed, that mad lust for land; Trust is about high-level corruption so where better to set that than in Sydney."

In Hammer's first three books, the main character is troubled journalist Martin Scarsden.

In Treasure and Dirt, homicide detective Ivan Lucic, who was mentioned briefly in Trust, takes over the reins, ably assisted by local investigator Nell Buchanan.

Hammer was keen to develop some new characters.

"There were a couple of things. One, in the first three books there was that emotional involvement of Martin and Mandy, they had skin in the game. Over the course of Scrublands, Martin changed, he was a different guy at the end of the book. In Silver we find out a little more about him, about Mandy, what shaped their early years. I couldn't write another story with those two in it, without that same level of emotional investment. Readers might have felt cheated if I just turned them into disinterested investigators.

"I just thought, this story lent itself to a homicide detective and I thought of Morris Montifore from the first three books but then I thought, no, he's too well defined as well. I wanted someone new."

Hammer, a former journalist with more than 30 years of experience, covering federal politics and international affairs, is still kind of blown away that he's now considered one of Australia's best crime writers.

His books hit the best-seller lists soon after publication, he's been to writers' festivals around the world - Val McDermid once selected him as "new blood" for her panel at the influential Theakston Old Peculier Crime Writing Festival

"It was kind of surreal," says Hammer.

"The festival is enormously influential, there I am with rock star authors like James Patterson, Jo Nesbo, Harlen Coban, Ian Rankin, Anne Cleeves.

"Val reads about 70-80 debut crime books a year and selects four new authors, it was such an honour, absolute fun but interesting and worthwhile too."

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He's writing full-time now, he didn't have any expectations beyond Scrublands.

"When I was writing Scrublands it was a bit of a hobby," he says.

"I'd already written two non-fiction books which were well received but I didn't make any money out of them, that's the reality.

"I turned to fiction because I didn't have the time or resources to write any more non-fiction and by the time I was getting to the end of Scrublands I thought this isn't too bad.

"But if you asked me then what I would be doing in five years time, I'd have probably said something like Scrublands will published and maybe I'll be working towards getting a second one published, but I'd still be working full-time.

"Scrublands only came out three years ago so it's four books in four years, I never would have predicted that."

He says lockdown has changed his daily routine - unlike most solitary authors you speak to.

"I think it's a hangover from my journalism days, where I used to travel a lot for work.

"I like writing when I'm on the move, my favourite place to write was on the train up to Sydney, or in cafes or on planes or in hotel rooms.

"I've been much more desk bound and in some ways that's good because there's less distractions, but I like distractions, depending on what part of the book I'm in."

He says there's another book on the go.

He gets very anxious between signing off on the final edits and the book being published.

"It helps if I have another project on the go, if my head is buried in the next thing.

"This one does feature Ivan and Nell, but the thing is I can't get away to look at locations, I'm looking at Google maps a lot."

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