The crescent-moon eyebrows; the rosebud mouth; the cantilevered cheeks. It's a portrait of St Francis, camp as a caravan. Painted by Leo Bensemann in 1937, the portrait, now held in Christchurch Art Gallery, represents an artist "absolutely going against the grain of everything else that was going on", according to his biographer, Auckland writer-curator Peter Simpson.

Simpson brings a 40-year obsession with the Christchurch artist to fruition with the release next week of his book Fantastica: The World of Leo Bensemann.

He believes Bensemann was at the epicentre of "everything bright that was happening in New Zealand" culture in the 1930s-40s and beyond. The epicentre was Christchurch, where "a sort of Art Club" (Bensemann's words) called The Group was set up in 1927, lasting for the next 50 years.

Its members included Ngaio Marsh, Rita Angus, Colin McCahon, Doris Lusk, Bill Sutton, Louise Henderson - and Leo Bensemann, who joined in 1938. But while the others were painting landscapes and portraits, Bensemann was "walking on the wild side", says Simpson. A guy with "a touch of the homme fatal" is how one of the artist's friends described him in a memoir quoted in the biography.

"He was a man out of step, a weird phenomenon that didn't relate to anything," says Simpson, who first met Bensemann in the mid-60s.

"That [the 1930s] was the time when they were first starting to define the notion that you could have a New Zealand painting and it would probably be landscape-based because the landscape was so unique and it hadn't been painted in the right way before. So Leo went off and did all these surrealist figures. Then, later in his life, he went and made landscapes, completely out of step with everybody else.

"I speculated about that - why was it? You could answer that in a number of ways. One was, he was a perverse sort of bastard, he enjoyed being non-conventional and not following everybody else. He was quite proudly independent. Then there was the fact he was genuinely odd. Part of it came from his being German, part of it came from his being gay or bisexual."

There: he has said it. Yet 10 years ago, when Simpson introduced a selection of Bensemann's work in a touring exhibition also featuring works by his great friend Rita Angus and his lifelong friend, the writer and Caxton Press editor Lawrence Baigent, "a great kerfuffle" ensued over that very issue: that the married man with four children had been gay or bisexual.

Simpson, one of Bensemann's sisters and Baigent's long-time partner (Baigent died in 1985), Robert Erwin, appeared on National Radio's Spectrum programme to talk about the show and its extraordinary trio of protagonists.

"The Spectrum lady asked directly if Leo was gay and Robert said yes," recalls Simpson.

"Then she asked if Leo and Lawrence had been partners. Robert said, yes, in the 1930s. Basically, Robert had outed Leo on the radio and it caused a great kerfuffle among the family [Bensemann's widow, Mary, was still alive at the time] because the notion that their father or husband was gay had never occurred to them, ever.

"Because of the kerfuffle that arose over that Spectrum thing, Robert backed off and then denied that this was the case and raced off to the Macmillan Brown Library at the University of Canterbury and put a 30-year embargo on all of Lawrence's diaries. He panicked because whoever contacted him from the Bensemann family was angry and the friendship with the family was very important to Robert."

With 20 years to go before the embargo is lifted, Simpson wistfully hopes he'll still be around to read them. "Those diaries will be a very important document," he says. "Those diaries will be published and become famous."

Simpson said his earlier drafts for Fantastica were circumspect about Bensemann's relationship with Baigent, a close friend since his teen years when they both lived in Nelson, then in the same houses in Christchurch from 1931-37.

"But my editor said, 'Peter, this won't do. There's a hole in the middle of this book and you're going to have to front up to it.' So I rewrote it. But I say there is no documentary evidence of the precise nature of their relationship. Some who knew them thought the relationship was sexual; others were sure it wasn't."

But some were staggered when, in October 1943, Bensemann married Mary Barrett, a woman he had met through his folk-dancing hobby. His friend, poet Allen Curnow, wrote a letter announcing that: "Leo by the way is married, 'suddenly', as the death adverts say, to Mary Barrett."

Simpson writes: "Curnow's tart remark implies that marriage was not an expected option for Leo, given his long-standing relationship with Baigent."

"After the marriage, he just switched and became a family man," he tells me.

Bensemann, whose parents were of Irish-German heritage, grew up in a home without books and he didn't do well at school.

His artistic skills were largely self-taught. As his art developed, stimulated by friends such as Baigent and Rita Angus, who shared a house with them on Cambridge Tce, the strength and diversity of his talent emerged.

He excelled in caricatures, cartoons, book plates, engravings and book illustrations and, in 1938, the year he became a member of The Group, he joined the Caxton Press, in partnership with Denis Glover.

There he became a master of typography, designing innovative covers and developing new typefaces for The Group catalogues and important Caxton publications, including works by A.R.D. Fairburn, Curnow, Frank Sargeson, Monte Holcroft, E. Mervyn Taylor, Ruth Dallas, James K. Baxter, Janet Frame and the Landfall series.

When Glover was forced out of Caxton, Bensemann eventually took over its management and later edited Landfall.

In the late 30s and early 40s, Caxton and The Group were central to Christchurch's bubbling artistic and literary set. Other centres such as Wellington and Auckland were relatively dormant.

"They had the whole national scene - they were it," says Simpson. "Then Glover went off to war [Bensemann, along with many of his peers like Angus and Baigent, was a pacifist] and it was never the same. Leo kept things going during the war but he couldn't afford to publish books - he published three books in four years. Glover came back with huge plans and published far too many books they couldn't sell and ran the place into the ground. In the meantime, he was getting pissed and drinking up the profit, absentee for days on end.

"Leo turned in desperation to Dennis Donovan, who'd worked as a boy at Caxton. He came back and put the whole thing on a business footing and gave Glover a job and a wage but he eventually sacked him."

In 1954, Caxton was running out of money.

"Glover went, Leo was overwhelmed, but they kept going and were faithful to people like Charles Brasch and Ruth Dallas," says Simpson, "but publishers were starting up in the North Island. "By the 60s, Caxton couldn't afford to publish books - they had to be paid so they could afford to print them for other people."

Bensemann edited 14 issues of Landfall between 1973-75 and retired in 1978.

At last he was free to paint full-time. His painting had stopped in the 50s - but, suddenly, in the mid-60s, he had started again. Gone were the surrealist figures. While many of his peers were discovering abstraction, Bensemann found joy in landscapes, particularly inspired by visits, with his painter friend Doris Lusk, to his old home turf of Golden Bay and Takaka.

His work, says Simpson, "started to simplify. It was not a literal picture of the landscape but extracting its essence. It lifted his painting into another order of quality."

Simpson first met Bensemann in 1965, when he'd joined the English department at the University of Canterbury and had started collecting New Zealand literature.

"I went into Caxton Press and asked about their stock. This burly bloke took me out the back and I bought a first edition of James K. Baxter. I realised later that he was Leo. Actually, earlier on, when I was an MA student, Lawrence Baigent was one of my teachers, a 60-year-old silver fox, we all admired him immensely. He used to hold parties for his MA students.

"I'd never seen a house like it - there were Leo and Rita paintings all over the place. One night, we were all sipping our cocktails and feeling like a million dollars and the door crashed open and this huge man, drunk, came bellowing into the room. It was Leo."

After living overseas for a decade, Simpson returned to the university and went to a one-man Bensemann show at the Brooke Gifford Gallery. He fell in love with Takaka Landscape, 1981, and bought it for £600. "I had this feeling that these were Old Master paintings. I fell so in love with it I felt I had to write about this guy."

The 1984 article in a large-format magazine called Untold was Simpson's first piece of art writing. He brings the magazine out and turns the pages of the feature which, as he proudly puts it, "goes on and on and on and on".

It was the first substantial critique of Bensemann's work. "I made a friend of Leo for life after that - not that he had much life left - and with the family for life as well. He was absolutely chuffed."

Bensemann died in January 1986, aged 73. His work had largely been ignored by critics and he was barely known in centres outside of Christchurch: the Auckland Art Gallery, to this day, holds no Bensemanns, although his stock has risen considerably in other public galleries such as Te Papa, which has six.

His appeal stretches to New York where a prominent collector, who owns two Bensemanns, contributed substantial funding towards the book so that Simpson, flying the flag for the artist, could have full colour and a hardback format.

For Simpson, the book is the product of determination. "There was a job to be done and nobody else cared," he says. "I thought he was a great painter, at the very least on a level with McCahon, Woollaston, Angus and that crew. Certainly earlier on, people just thought he was weird and he played up to that, put himself in the role of being the outsider. He cultivated this kind of sinister persona.

"Then he went off a bit, and then he suddenly started painting landscapes, wildly out of key with everybody else. I couldn't get a show that was just Leo works off the ground so I hitched his wagon to Rita's star to get people looking at his work. Now, 10 years later, I was able to do the big job."

* Fantastica: The World of Leo Bensemann (Auckland University Press $75) is out on Tuesday. An exhibition, Leo Bensemann: A Fantastic Art Venture, is at Christchurch Art Gallery from February 11-May 15.