In a bitter dispute that exposes the limits of the law, a young man who died without a will has unwittingly left his entire estate to his estranged wife - even though the couple had already split their assets and had new partners.

The man's grieving family has no rights to the assets.

"It still astounds me now," said his father, who described his "total shock" at the sudden death of his 31-year-old son.

Before his death, his son told him he had met the woman who was "the one" and the new couple spent the last 15 months of his life together.

A missing signature is at the heart of the legal battle. The man's estranged wife, who wanted to remarry, had served signed divorce papers on him months before his death but he had not signed them.

If he had been divorced or had left a will, his estranged wife would have no claim on the estate, which includes a house, superannuation and a death benefit. But because they were still legally married and they had no children, the state Succession Act says she is entitled to the whole of the estate.

After the couple split, the man paid his estranged wife $35,000 under a property settlement which allowed him to keep the matrimonial home. He had built the house on a block of land he bought before he was married.

"She instigated the divorce proceedings - she got her money and moved on," the father said.

The father will be entitled to have funeral costs and potentially some debts paid but he has no further rights to the estate, although the parties may come to an agreement.

The lawyer for the man's father said the estranged wife was "getting a second, uncontested bite of the cherry".

"It's not something you can blame any parliament for, because it probably has not been brought to their attention previously," he said.

"The way that the law should be ... is that if there has been a property settlement between the parties and there has been no reconciliation of the relationship, then clearly the surviving spouse has received her or his fair share of the estate."

But the lawyer for the estranged wife said "there was a lot to this that doesn't seem fair to this girl either" and she had done nothing wrong. He said she had a legal right to the estate and had found herself the subject of a "hatred campaign".

The woman was not legally represented when the property settlement orders were drafted. Her solicitor said the orders could have precluded her from entitlement to his estate, but they did not. Her late husband could also have made a will or signed divorce papers, but he did not.

The man was living with his new partner at the time of his death and they planned to buy a house and have children together. But the Succession Act only gives de facto spouses an automatic right to part of the estate if the couple had lived together for at least two years or had a child together.

The man's new partner said his father was left "heartbroken" over the dispute. In a last-ditch legal bid, she will ask the Supreme Court to grant a "family provision order" to give her a share of the estate.

"I'm not by any means money-driven but I think that [her late partner] would want what's right to take place," she said.