Hidden away on a shelf at the Mitchell library in Sydney sat what Dr Michael Walsh thought was a chunky volume from the New South Wales state library's vast collection of colonial manuscripts.

He pulled it from the shelves only to realise it wasn't a book but a box, containing two notebooks. He flicked through the first pages which contained, in Walsh's words, "a lot of doodles" – but on page seven things got interesting.

"A short vocabulary of the natives of Raffles Bay," it said. Walsh had just rediscovered a guide to the Indigenous languages used near a British settlement on the coast of the Northern Territory, written by the Victorian colonialist Charles Tyres. The text had been unknown to modern academics.

"At that time I figured, well, probably no one knows about this because I only stumbled across it by dumb luck," says a modest Walsh. But the notebooks form part of a huge array of documents uncovered at the state library.

A two-year research project, headed by Walsh, has sifted through 14km worth of colonial manuscripts that shed light on 100 Indigenous languages, many of which were considered lost before the finds.

Walsh describes another of the discoveries he's particularly proud of, a 130-page trilingual dictionary in German and the Indigenous languages of Diyari and Wangkangurru from the north-east of South Australia. Diyari has been undergoing a revival in an attempt to keep it in active use.

"Compared to some of the other resources that might be as small as 20 words, this is quite a substantial addition … so to suddenly get 130 pages from the late 19th century popping up is quite a find," Walsh says.

The jubilation of unearthing such a document is beset by a sobering reality. The federal government estimates that 145 Indigenous languages are still spoken in the country, but an overwhelming 110 are threatened with extinction.

These documents, collected in part to harvest knowledge amid attempts to exert control on Australia's Indigenous population, will now help to preserve that culture.

"There is a certain irony there, I guess," Walsh says. "One harsh view would say that the people who were collecting this stuff were colonialists who were basically intent on stealing Aboriginal land ... settling the country and opening it up to pastoralism.

"Whichever way you look at it their main intention was not necessarily all that benign for Aboriginal interests.

"In some instances though, [there were people] like Charles Tyres, who did have a reputation for being quite sympathetic towards Aboriginal people and treating them with respect, whereas the others it's not quite as savoury a story."

A map from 1940 showing the Indigenous peoples of Australia. Photograph: State library of NSW

Walsh, a linguistics professor at the University of Sydney, obviously takes great joy in the academic rigour and investigative nature of the research but he is keenly aware that these discoveries have a tangible, pragmatic end.

"People, especially in New South Wales, will tell me that once I regain my language through language revitalisation, I also regain my identity, not just as a Koori … but I'm a specific people," says Walsh.

"Some of those people also say that once they've regained their identity, they sort of improve as people. There are people who have been dysfunctional with the police, getting drunk, not being able to hold down steady employment, they say that it's the language that brought them back into the world – because of that regaining of identity."

The research phase of the project is drawing to a close, with the next step being to disseminate the findings to the relevant Indigenous communities and language experts. The library hopes, with the correct cultural approval, to digitise the discoveries to make them more easily accessible. Walsh says speakers of some of the languages are scattered around the world.