It seems unlikely that the wheels of justice would have budged at all after the Pike River mining disaster in New Zealand, which killed 29 men in 2010, had it not been for the actions of the grieving families. This sobering true-life drama is a blow-by-blow account of their years-long fight for justice. The story is told with restraint, in meticulous detail – possibly too much detail – without any Erin Brockovich-type feelgood emotion, never losing sight of the heartbreak and devastation.

The film begins with agonising scenes before the accident: the miners get ready to go underground, good-naturedly ribbing each other – a familiar morning routine – with no clue of what’s coming, although in cinematographer Gin Loane’s foreboding camerawork the spectacular landscape of forests and mountains looms threateningly. In the hours after the first explosion rips through the mine, the company’s CEO (Jonathon Hendry) confidently says there is enough air underground to last several days. “But we’ll have them back before then.” The lies told to families were staggering.

The film centres on a friendship that develops between Anna Osborne (Melanie Lynskey) and Sonya Rockhouse (Robyn Malcolm) during the weeks and months they sit through royal commissions and public inquiries, clutching photographs of their dead. Anna’s husband and Sonya’s 21-year-old son are among the victims; one of their goals is simply to recover the bodies from the mine.

At times, it feels hopeless. But eventually the victories come, sometimes from unlikely quarters, such as the family-run concrete business that refuses to supply concrete to fill the entrance of the mine. There’s a surprise cameo waiting at the end of the film. I watched the final scenes thinking that the film-makers did solid work finding someone to play a certain superstar New Zealand politician – but it turns out she did the job herself.

• Pike River is on digital platforms from 9 February.