"I am so weak that I could eat a piece of man."

One of Tasmania's least-known convict sites is to be found in roughly the spot where Marcus Clarke ill-prepared generations of readers for one of the set-pieces of Australian fiction.

Follow a dirt track 23 kilometres off the Lake Leake Highway, cross a foul-smelling creek, climb a steep bank, and you emerge into a clearing reminiscent of an Inca encampment: half a dozen stone buildings, roofless, with tall gums thrusting up through the foundations. A paved path leads to a well and, further on, to a headstone of red rock carved with the name of Thomas Collins who died in 1843, aged 36, after a stone crushed his skull.

These ruins are the remains of the Long Marsh Dam, a project by local landowners to harness free convict labour and irrigate 540 acres of farmland from the Macquarie River. At one time 279 men lived here. For 18 months they toiled, often shoeless, their feet wrapped in cloth, hand-digging and transporting huge slabs of bluestone to erect a dam 18 metres high. Then in 1844 arrived an order from London that the landowners had to pay the convicts (4/8d per day for mechanics, 2/2d for labourers). The landowners refused and so, elated, the convicts downed tools, leaving concealed in the bush a remarkable and rarely observed freeze-frame of Van Diemonian history.

By unusual coincidence, it was in the vicinity of this dam where Marcus Clarke had his fugitive convict Gabbett begin to eat four of his companions, ten days after their escape from Port Arthur. Only, Clarke - a Melbourne journalist who paid a single visit to Tasmania in 1870 - betrayed an insulting ignorance of the landscape into which he packed his famished felon (Gabbett's deterioration was based on Alexander Pearce's journey through the very dissimilar west coast). "All round is the fruitless, shadeless, shelterless bush," Clarke wrote in For the Term of His Natural Life, so adding the landscape itself to Gabbett's list of victims. It's a calumny that remains hard to shift in many a mainlander's imagination. Far from being the "grey wilderness" of Clarke's fictional depiction, Tasmania's east coast was then (and still is) in the view of James Boyce, "one of the most hospitable and benign environments for human habitation anywhere in Australia".

On 1 January 1856, 12 years after the Long March Dam was abandoned, and half a century after the colony's foundation, the settlement of Van Diemen's Land - a name, wrote Trollope, "harsh with the crack of the jailer's whip" - achieved self-government under the more euphonious title of Tasmania. "‘Tasmania' represented the new society the free immigrants sought to superimpose on the convict homeland of Van Diemen's Land," writes Boyce, whose contention is that the essential character of the original colony never vanished, "but by the edict of an embarrassed ruling class it went underground." Rather like the Long Marsh Dam, Van Diemen's Land persisted out of sight of Tasmania: a "cultural museum of the pre-industrial era where convicts and their children were to remain the large majority of the population for decades to come".

A sister-book to Tom Keneally's excellent The Commonwealth of Thieves, about early New South Wales, Van Diemen's Land (Black Inc., 400pp; $49.95) is a valuable reappraisal of the first 50 years of Australia's second-oldest state. It does not radically alter the story of what we know so much as enrich and qualify it: here giving fresh emphasis (e.g. to George Augustus Robinson's duplicity towards the Tasmanian Aborigines), there bringing to the fore what might have been forgotten (e.g. the part played by ex-cons from Tasmania in the foundation of Melbourne), and everywhere adding sinew and flesh to what has often resembled the limiting cross-hatch of a Gothic cartoon. Boyce's achievement is to rescue his island's narrative from the small middle-class elite who sought to recreate a "Britain of the south" - and whose Georgian mansions survive to tell their tale - and to put it in the hands of the overwhelming majority: the convicts and ex-convict stock-keepers, shepherds and bushrangers who were able to forge a close, very un-English identity with the land, but whose bark huts have rotted away.

Clarke's geographical error encapsulates the misconception that has clouded our understanding of the effect of Tasmania's landscape upon the 72,000 convicts transported there. In this "pernicious national narrative", suggests Boyce, Van Diemen's Land has been recast in the horrid mould of early New South Wales: Clarke's grey wilderness of "cruel scrub" striking an echo of surgeon-general John White's 1790 reaction to Port Jackson as a waterless, hostile hell-hole "so forbidding and so hateful as only to merit execration and curses". But the experience of those exiled to the southern colony was dramatically different. In Van Diemen's Land, the land was not the lock but the key. The bush spelled freedom, even redemption.

All but the blindest curmudgeons agreed: the island's landscape was arrestingly beautiful. Boyce manages to convey how it was "a veritable Eden" in plenty of other ways. "Nature could be called on for ‘free' food, clothing, shelter and heating, as well as being a source of supplementary income." Equipped with no more capital than a dog and an axe, a convict could work wonders. The well-watered, dingo-free grasslands bustled with easy-to-catch kangaroo (tasting like venison) and echidnas (like goose), and offered a nutritious diet to men more accustomed to a hard, black substance "served out as beef, but commonly called Old Horse". The convict James Grove spoke for many when he named it "a bed of roses" compared to life in Britain, where there were hunger riots and one in nine existed on poor relief. "It's being so contrary a life to that which we have been used to ... I am - as must anyone be - astonished at it." The prospect of going back to England left Grove feeling "unaccountably indifferent". The abundance of fresh food and water, plus an absence of epidemics, made the colony, according to Boyce, "one of the healthiest outposts of the British Empire". In calm, lucid prose he drives home how the convicts enjoyed privileges and rights simply not available to their counterparts in Britain. Even as late as 1826, Lieutenant-Governor Arthur was battling "to make transportation a punishment which, at present, it certainly is not". Hence Port Arthur.

Arthur's arrival (in 1824) put a stop to this fluidity, in which the bountiful grasslands were treated as "common ground". Van Diemen's Land, by then, had become a parallel destination for that contingent of relatively wealthy free settlers who desired this Eden for themselves. Their land-grab set these new immigrants in brutal opposition to the convict occupiers as well as to the Aborigines whose hunting grounds these had been, as Boyce nicely puts it, for three times longer than Homo sapiens had inhabited Britain. By 1831, the bushrangers and most Aborigines had been killed or evicted, and the ensuing property boom swiftly altered the usage and contours of the landscape. Fences sprang up; introduced crops replaced the wild grass (plus rampaging colonial weeds like blackberry, thistle and gorse); the thylacine and emu vanished before the onslaught of the dog, the sheep and the cow. As well, for reasons more complex, the original native population died out.

It is hazardous to build a theory on a midden of hearsay and conjecture, but Boyce's explanation for the speed of the Aborigines' demise after 1824 is utterly convincing, although his theory, it must be said, is also "without [the] direct evidence" for which he chastises other historians. Bluntly put, these historians have "significantly reduced" the easy slaughters that occurred in the four years prior to the richly documented period (1828-31) of the Black War. "In reality, the British were fighting an already defeated enemy."

The rawest part of Boyce's book is his 52-page appendix: ‘Towards Genocide: Government Policy on the Aborigines, 1827-38'. It juts out brooding and uncomfortable, almost too important for an appendix. Boyce's careful manner drops as he catalogues how the Aborigines came to be removed from their ancestral lands "by force or trickery", and without the approval or knowledge of London. Whether this "government-sponsored ethnic clearance" was as deliberate as Boyce maintains or, just as likely, was a product of the "hopelessly contradictory nature of government policy", the trauma of the outcome is not in dispute. There can be few more painful remarks than the one recorded by George Augustus Robinson on Flinders, the island which succeeded in operating for Aborigines as Van Diemen's Land was intended to serve for convicts: a place of prison and exile. "Why keep us here to starve. We don't want to live here. Let us go to our own country and we can live."

This is a first-rate history. Any criticisms one might level are minor, but should, perhaps, be aired, given the cannibalistic fury directed towards anyone venturing into this terrain. Boyce, like Lyndall Ryan and Henry Reynolds before him, is reassuringly fallible in his transcription: Mrs Prinsep's phrase "girls in their pattens" (wooden clogs to navigate the Hobart mud) is wrongly rendered as "girls in their patterns". In emphasising the bounty of the place, Boyce ignores the genuine conditions of famine that existed in Port Dalrymple in 1806-07, and which precipitated the first crossing of the island. Likewise, in his eagerness to exempt the Aborigines from epidemics, he overlooks the Flinders Island catechist who was told by Aborigines that "their numbers were very much thinned by a sudden attack of disease which was general among the entire population previous to the arrival of the English, entire tribes of natives having been swept off." In a spirit of the purest selfishness, I would have welcomed a reference to the brief period in the 1820s when convict-harvested whale oil from Oyster Bay, where Gabbett was hobbling towards (and where I write this), illuminated the streets and studies of London. But as I say, small quibbles when set against the clarifying and very Tasmanian light that Boyce sheds on his colony's birth, along the way reminding us that Gabbett, like a fair few commentators after him, had no need to eat his companions, only some of his words.