An elderly grandmother who believed her son died childless has been ordered to hand over most of her home after a DNA test revealed she should not have pocketed his $485,000 estate.

Norma Larkin-Britt, 87, sold her late son Robert's unit in Sydney's west and funnelled the proceeds into a string of property purchases in Bonny Hills, a coastal town 18km south of Port Macquarie, before later buying a four-bedroom home in the nearby village of Lake Cathie, where she now lives.

But Supreme Court judge Francois Kunc found she failed to properly investigate whether her son had a biological child.

That child, Justin Sylvester, who was born in the late 1970s, has now been awarded a staggering 85.8 per cent stake in her million-dollar home.

Robert died in 2014 after he was found unconscious in his Wentworthville unit by friends and later declared dead in hospital. He had no partner and did not leave a will.

The court heard that while Norma had long known of Justin's existence, she had come to believe that he was only her son's stepson.

However, years later, she reached out to him with a stunning message on Facebook: 'I may be your grandmother.'

Norma went on to form a close bond with Justin and his partner Rebecca Best, before the relationship dramatically broke down, setting the stage for a bitter legal fight.

Justice Kunc found that Norma's evidence was clouded by bitterness and heartbreak over their falling-out, as well as her struggle to accept the DNA result.

In tense exchanges, she insisted Justin was still only her 'step-grandson' - despite the genetic test proving otherwise - admitting: 'In my heart I feel as though he isn't.'

She told the court her disbelief was fuelled by the animosity of the lawsuit, saying no one in her genetic family would do 'what Justin is doing to me'.

Justin's birth certificate lists Robert as his father, with his name originally registered as Justin Lloyd James.

A key piece of evidence in the form of a card helped unravel Norma's claims that she was not aware of the paternal link, with the court hearing she once congratulated her son and his partner on the birth of 'your baby son'.

She wrote, 'Congratulations on the birth of your baby son with lots of love to all three,' before later visiting the couple and newborn Justin at their Rooty Hill home.

The judge accepted that at the time she sent the card, she believed she was congratulating Justin's biological parents - a finding that cut against her later claims.

After his parents split in 1979, Justin grew up largely without his father, not seeing him again until he tracked him down as a teenager.

In 1996, at age 16, he met with Robert after an aunt had arranged a reunion.

But the long-awaited reunion soon unravelled. Within a year, an argument over a gift - a jacket and shoes - turned physical, with Justin then claiming his father pushed him.

From there, their connection fell apart, dwindling to a handful of phone calls before they lost contact for good.

The court heard Robert later told his mother about their falling-out and, in the mid- to late-1990s, said to her that 'I don’t think he is my son'. 

From this point onwards until about 2014, when Robert died, Norma settled into the 'stepson theory', as reinforced by Robert's comments and a hospital questionnaire she found in his unit after his death where he ticked a box saying he had no children.

However, the judge noted that Norma did not know one way or another.

Justice Kunc ultimately found that Norma did not act reasonably in administering the estate, ruling she should not be excused for distributing it to herself without properly checking whether her son had a child.

He wrote in his published decision that 'between 2016 and 2019 Norma accepted that Justin was Robert's biological son.

'Even assuming that she was ignorant that this status entitled Justin to the estate, I find that Norma did not act reasonably because a reasonable person in her position would have then sought legal advice. She did not do that.'