The High Court has torn up a prenuptial agreement between a wealthy Australian property developer and his online bride, who was pressured into signing the document after he threatened to call off the wedding.

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.

The High Court tore up the prenuptial agreement, described by one solicitor as the "worst" she had ever seen. Credit:Karl Hilzinger

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.