When Harold Haig was 10 years old, a man in a suit came to visit. "He said to me, 'Would you like to go to this wonderful place called Australia where the sun shines all day every day and you pick oranges off the trees, live in a little white cottage by the sea and ride a horse to school?'" remembers Haig, who is 73 but looks younger, with Pete Postlethwaite cheekbones and flowing white hair. "While I was letting this sink in, he added, 'Well, you know you're an orphan, your parents are dead, you've got no family, you might as well go.'"

Haig was one of 7,000 children from British care homes who were shipped mostly to Australia and Canada between the second world war and 1967. The scandal of the lies and abuse suffered by these child migrants was exposed thanks to the tireless work of Margaret Humphreys, a social worker from Nottingham, who, in 1987, took it upon herself to help them find their families. As Oranges and Sunshine, a moving new film by Jim Loach – son of Ken – shows, Humphreys defied death threats to discover the truth about these former child migrants and their past lives. When Haig begins to talk, it is eerie because his softly spoken words and manner exactly resemble those of Jack, a traumatised former child migrant in the film who is played by Hugo Weaving. The British-Australian actor met and talked to Haig about his experiences before taking the role.

Apart from the man in the suit talking of oranges and sunshine, Haig barely remembers anything of his childhood in Britain. "Because of my lack of memories, I may as well have been born in Australia when I was 11 years old," he says, bleakly. He was sure he had a sister called Marie, but he could not remember anything at all about his mother: no image, no voice, no smell. "Just a blank. An absolute blank."

Surrounded by other "orphaned" children, the voyage to Australia was an adventure ("we ran riot"). When Haig arrived, he was dispatched to a Church of England boarding school in Melbourne. Other child migrants were less fortunate, as Oranges and Sunshine reveals through the story of Len, played by David Wenham. Many ended up in the care of the notorious Christian Brothers where they were treated as slave labour and suffered horrific physical and sexual abuse. One victim told an official inquiry that his Christian Brother carers competed to become the first to rape him 100 times.

Haig escaped such trauma – he would be beaten with a strap if he did anything wrong – but, as he says: "The thing missing in an institution for children is that there is no love. You get punished but there is no one there to put their arm around you and say it's OK." One of many powerful scenes in Oranges and Sunshine is when the character based on Haig falteringly explains how he feels: "There's an emptiness in me. There always has been and I think the only thing that could fill it was her, my mother." Haig says something similar when he talks of how he married, had three children and established a successful signwriting business: "Anyone would've thought there's a fella who's got everything, but it was like I had a block of ice inside me. I felt empty. I knew I was missing something. I couldn't work out what it was. And there was this feeling – I didn't know who I was. I didn't know where I'd come from. I didn't belong to anybody. I was in this void."

In the 1960s, Haig sank into a deep depression. He was prescribed antidepressants, saved them up and swallowed them all. "I wanted to die. I wanted to go to sleep and not wake up to get rid of this pain, this emptiness," he says. His wife, normally a good sleeper, woke up and saved his life. He wishes he hadn't tried to take his life at home, while his children slept.

The "beautiful" younger sister he was always convinced he had eventually traced him through the Salvation Army. Marie had been separated from their mother and Haig, and raised in care homes in Britain; unlike Haig, she remembered her sibling. One day, in 1987, Marie told him she was coming to Australia with a social worker, Margaret Humphreys, who she wanted him to meet. Haig, by then divorced and wandering the Australian outback ("I don't know what I was looking for"), was unimpressed. "I'd seen a lot of social workers and I had no respect for any of them," he says.

While Oranges and Sunshine shows Humphreys struggling to win the trust of some child migrants, Haig quickly came to respect her. She was the first to raise the possibility that Haig had been told a terrible untruth – that he might not be an orphan after all. "I didn't think anyone would be so cruel to tell you that sort of a lie," he says. He is amazed by Emily Watson's performance as Humphreys in the film. "I could've been watching Margaret," he says.

Haig visited Britain for six months in 1989 to get to know Marie, who passed away 14 years ago, and to help Humphreys track down his mother. With so little record-keeping by the authorities, still in denial over the scale of the trauma they created, it took another few years for them to get confirmation that Haig had not been an orphan. His parents had separated during the war, and with two children, no benefits and no relatives nearby, his mother had been forced to give up her son and daughter.

Humphreys discovered Haig's mother had lived two miles from where he was kept in homes (eight institutions in 14 months before he was "deported" – as the former child migrants say – to Australia) and had died just a year before he first visited Britain. The belated release of more suppressed information 10 years ago also helped Humphreys, who was awarded a CBE this year, finally identify Haig's deceased father.

No photographs remain of his mother, and Haig will forever wonder why he was given up and whether his mother tried to find him. As Oranges and Sunshine shows, parents were often deceived by the authorities and told their children had been adopted or even that they were dead. "Mothers went to their graves never knowing that their children were still alive, and happy, and well," says Haig. "It's criminal. I don't know what worse you can do to people."

Why did this happen? For the British authorities, a one-way ticket to Australia was cheaper than looking after children in care homes. For the Australian government, petrified they would be overrun by Asian immigrants, white children were ideal fodder for the racist "White Australia" policy.

In 2009, the Australian prime minister Kevin Rudd apologised to child migrants. "It's a day we'll never forget," says Haig, who is secretary of the International Association of Former Child Migrants and their Families, and is still good friends with Humphreys. Gordon Brown followed with an apology on behalf of the British government a year later.

The trauma of all these state-sanctioned lies and abuse has left a long, scarring legacy. Haig is still estranged from his two daughters who felt deserted when his depression destroyed his marriage. "They think I abandoned them, and in many ways I did. I had trouble looking after myself," he says, anguish in his voice. He has since been reconciled with his son, and he hopes the film might yet bring him back together with his daughters.

"What Margaret did for me and for thousands of child migrants is to give us back our lives, give us back our identity, and shine a light in where there was just darkness." Where would he be without Humphreys? "I have my doubts about whether I'd be here alive," he says. "You should ask, where would all of us be?"