The man at the centre of the case, who owned assets worth more than $18 million, died in May 2014 during drawn-out litigation over the agreement.

Two of his children, acting as executors and trustees of the estate, took over the court battle.

On Wednesday, the High Court ruled the agreement, and a similar post-nuptial agreement, should be set aside on the basis of unconscionable conduct.

The Federal Circuit Court had ruled in 2015 that the agreements were not valid but the decision was overturned by the Family Court last year. The High Court decision upholds the earlier ruling.

The court said the couple - given the pseudonyms Mr Kennedy and Ms Thorne - met in 2006 on "a website for potential brides".

"At the time, Ms Thorne, who was an eastern European woman, was living in the Middle East. She was 36 years old. She had no substantial assets," five of the seven judges, including Chief Justice Susan Kiefel, said in a joint judgment.

"Mr Kennedy was a 67 year old Greek Australian property developer. He had assets worth between $18 million and $24 million. He was divorced with three adult children."

The couple married just over a year later, months after Ms Thorne moved to Australia to live in Mr Thorne's "expensive penthouse", the joint judgment said.

The four-bedroom, five-bathroom property had "multiple balconies and [an] upper roof deck with pool" along with "marble flooring, decorative cornicing, gold leaf decorative fittings, a chandelier, gold plated tap wear, and murals on some internal walls and ceilings", the Federal Circuit Court said in its 2015 judgment.

Ten days before the wedding in September 2007, Mr Kennedy took Ms Thorne to see an independent solicitor about the terms of the prenuptial agreement, as is required by law. He had told her early on in their relationship that "you will have to sign paper" or the wedding would not go ahead because "my money is for my children".

The independent lawyer told Ms Thorne: "It is the worst contract I have ever seen. Don't sign."

The agreement said Ms Thorne would get nothing if the couple separated within the first three years of marriage. If they separated after this time and the couple did not have children, Ms Thorne would receive a single lump sum of $50,000 - an amount described by the lawyer as "piteously small". The use of a $500,000 unit would be provided if the couple did have children.

The lawyer said she had "significant concerns" Ms Thorne was only signing the agreement so the wedding would not be called off.

The couple separated in June 2011, less than four years after their wedding. Ms Thorne started legal proceedings in April 2012, seeking to have the pre- and post-nuptial agreements set aside. The High Court agreed with the Federal Circuit Court and said the agreements should be torn up.

"Mr Kennedy took advantage of Ms Thorne's vulnerability to obtain agreements which ... were entirely inappropriate and wholly inadequate," the joint judgment said.

The Federal Circuit Court will now consider Ms Thorne's application for a $1.1 million property adjustment order and a lump sum of $104,000.