Historians believe a nitrate film unearthed in Sydney could be the original version of the only documentary of Phar Lap filmed while the horse was still alive.

The rare footage of the champion racehorse was recently given to the National Film and Sound Archive of Australia (NFSA) by the Australian Turf Club after it was discovered in a canister by Sydney racing personality Max Presnell.

The 10-minute film includes vision of Phar Lap at trackwork and interviews with his trainer Harry Telford and regular jockey Jim Pike.

It also features his 1930 Melbourne Cup win and the only known footage of his Randwick Plate victory a year later, the last time Phar Lap competed in Sydney.

The NFSA already held prints of the film `The Mighty Conqueror but they were safety copies and the location of the original nitrate was unknown.

"The acquisition of this 35mm nitrate film print of The Mighty Conqueror is of particular significance to our sporting film holdings as it is unique to any collection, both in Australia or abroad," said NFSA chief executive Michael Loebenstein.

Australian Turf Club Heritage Society officer Margaret Helback has described the film as "one of the more rare pieces of racing history we have ever come across".

The footage will be shown during Saturday's George Main Stakes meeting at Randwick.