The Hunter Valley woman was just about to turn 53 when she discovered her biological father was not the man she had called 'Dad', but the man who delivered her as a baby, Dr Norman Walker.

Instead of using sperm samples provided by medical students, the OB-GYN had used his own.

"I knew him growing up really, really well," Ms Darroch, now 57, said. "He was in my life right from the day he delivered me.

"I was his goddaughter. I have a younger brother and sister from him as well."

Ms Darroch, who grew up in South Africa before moving to Australia, said she was 16 years old when Dr Walker died. But it was during a conversation about genetics with workmates about six years ago that something "clicked".

"It just made me think of him and his blue eyes, and my blue eyes - both of my parents have dark eyes," she said. "I started Googling."

Dr Walker had written two novels under a pen name, and a search for them online led Ms Darroch to a comment from a woman living in Ireland. She said he had been her biological father - a sperm donor.

When Ms Darroch looked through old letters and photos he had sent her years earlier, she noticed the striking resemblance between him and her youngest daughter.

When confronted, her mother admitted they used a sperm donor. They were told the sample would be taken from a medical student and no one would ever know.

By then, her legal father had died.

Then three years ago, Ms Darroch's daughter received an ancestry kit for Christmas.

"She tested, and a whole heap of people started matching with her as close relatives," she said. "She was like, 'Mum, I think these are your siblings'. One of my half-siblings had given everyone in his family a kit, and, oh boy, did that cause some problems... I tested, and we all matched as siblings."

Ms Darroch has since connected with siblings in the UK, the US, New Zealand, South Africa and Australia.

"The youngest one to pop up is 14 years younger than me - so it would have been going on for 15 years, at least, but we know we weren't the first," she said.

A nurse who worked with Dr Walker confirmed he had helped "at least 100 families" - many with multiple children, and sets of twins.

"I think he has a lot of kids out there. Hundreds."

It had been a rollercoaster.

"When I first found out, I thought it was funny - a really fun story to tell at dinner parties," she said. "It has only been since speaking to all the other donor-conceived people, meeting my siblings and seeing the impact it has had on them and the pain it has caused them, that I started to wonder what could have been if I had known. I feel cheated. I didn't get to meet my paternal grandparents.

"A lot of people really struggle with being donor conceived.

"There usually comes a time when you realise what you have lost. "It might hit during adolescence. It might hit when you have your first kid. Or grandkid.

"Having known my biological father well, and knowing what sort of a person he was, he definitely wasn't a narcissist. He wasn't out to make money. He did want to help people. But his ethics were screwed."

Ms Darroch, a clinical psychologist, shares her story on Insight on SBS on Tuesday in the hope it might help her colleagues better understand how to treat the trauma associated with being donor-conceived.

"You can get a lot of push-back from people you've gone to for counselling," she said.

"The push-back we get is very similar to adoptees - 'You were very lucky to be adopted and you should be grateful you were so wanted'; 'Your parents went to a lot of trouble to get you', 'That doesn't make a family, it's who you're raised with' etc.

"It is disenfranchised grief. You're supposed to be grateful, you're supposed to be positive - not feel grief for the fact you had all of your relationships severed, or that you were lied to. You are supposed to be more understanding... It's all that toxic positivity.

"What I am trying to do is get them to see things a bit differently.

"This is trauma we are talking about."

See more on this on Insight's DNA Secrets at 8.30pm Tuesday or on SBS On Demand.

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