Non-professional American "street photographer" Vivian Maier didn't just hide her candle under a bushel, she also attempted to hide the bushel. Keeping both past and present to herself (she once claimed to be "a sort of spy", and her "French" accent is the source of much debate), the habitually private Maier was an extraordinarily sharp shutterbug who left thousands of striking images undeveloped, unseen even by herself. Having stumbled upon a box of her negatives at auction in 2007, John Maloof set about piecing together the untold story of this as yet undiscovered talent. The film follows its celluloid leads to the doors of those for whom Maier worked as a nanny (her primary employment) who recall her eccentricities as bordering upon psychosis.

From the obsessive hoarding of unread newspapers to the sometimes violent mistreatment of her young charges, Maier had "a dark side" which attuned her to the follies and failures of mankind – her brilliantly framed low-angle Rolleiflex portraits of poverty are a strange mix of the voyeuristic and the empowering. Mary Ellen Mark cites Robert Frank and Diane Arbus as reference points, but Maier's monochrome "crime scene" impulses seem closer to Weegee. Just how thrilled Maier would have been by all this remains unknown; the recent BBC documentary Vivian Maier: Who Took Nanny's Pictures? offered contrasting views of the ad-hoc curation and exploitation of her work. Either way, it's a fascinating story which continues to raise more questions than it answers.

• Comments have been reopened to time with the film's Australian release.