​A documentary about the Nazi concentration camps that was started in 1945 but left unfinished for decades has been completed and digitally restored by Imperial War Museums (IWM) in Britain. German Concentration Camps Factual Survey will have its Canberra premiere at Arc Cinema to commemorate the 70th anniversary of the liberation of the Belsen concentration camp.

IWM's head of digital collections, David Walsh, says the film was started  in April 1945 after the concentration camps had been uncovered. On the orders of the Supreme Headquarters Allied Expeditionary Force,  producer Sydney Bernstein assembled a team at the Ministry of Information that included editors Peter Tanner and Stewart McAllister, writers Colin Wills (an Australian journalist) and Richard Crossman (later a Labour MP) and director Alfred Hitchcock, who returned briefly from working in the US to work as a treatment adviser.  Footage was obtained from British, Russian and US crews who filmed in different camps but  after some initial work, the project was put to one side.

Walsh says, "The film's time had passed."

The original intention had been to show Germans the terrible things that had been done in their name. But by September of 1945 the political climate had changed in favour of supporting Germany's recovery rather than rubbing people's noses in the Nazi atrocities, he says.

The silent rough cut of five of the planned six reels of footage  and 100 compilation reels of unedited footage was acquired by IWM as well as a script for the planned voice-over commentary and a detailed shot list for the completed film.

In the 1980s an incomplete version using unrestored footage  titled Memory of the Camps with a narration by actor Trevor Howard was released in what Walsh says was "a mannered performance".  In 2008, staff at IWM were in the process of restoring that film but realised they could use the footage, production and editing notes and digital technology to make a film as close as possible to the original vision.

"The final reel had been missing from the film shown before – it was not edited at the time of production."

It was renamed – German Concentration Camps Factual Survey  was the title  originally listed in the Ministry of Information Catalogue of Films for Liberated Territories – and the commentary was rerecorded by actor Jasper Britton.  An effects track was created, blending the existing synchronised sound recordings made at Belsen with authentic Army Film and Photographic Unit recordings made on the battlefields of Europe.

No music had been written for the film so none was added: the images, sounds and narration were allowed to stand alone in this newly remastered version.

"One of the things that makes it so powerful is that the footage has so often been seen with an archival veil over it – scratches, markings," Walsh says.

Working with the original camera negatives meant the picture quality could be vastly improved, adding an immediacy to the grim sights depicted in the footage.

The film had its Australian premiere at last year's Melbourne International Film Festival and the Canberra screening is the only other one presently planned for Australia.

The screening will include a newly filmed introduction and epilogue and will be followed by a Q&A with author Helen Lewis, daughter of Allied armed forces cameraman Mike Lewis who filmed at the Belsen concentration camp.

Walsh says one of the people featured in the new footage is a survivor of Belsen whose reaction to the film is measured and matter-of-fact. That's not uncommon among survivors who see it, he says: while others may be shocked, for them it is simply a record of what happened.