In the new book 'Universal Judgement', Nuzzi used 3,000 confidential documents to support his claims of the Vatican rapidly losing money. His book claims the Vatican is in danger of defaulting by 2023 if something does not change soon to balance the books.
Nuzzi writes that the Vatican lost nearly €44 million last year as its vast property portfolio went into the red for the first time.
Speaking to The Times newspaper Nuzzi said: "Pope Francis's reforms of the Vatican's finances are failing, like a plaster placed on a hemorrhage".
The book documents the establishment of an emergency task force in the Vatican last year to ward off a financial meltdown as 2018 losses rose to €43.9 million form €32 million the year before.
One of the main causes according to documents is the drop in donations, known as Peter's Pence after the reputation of the Church has been marred by sex abuse scandals.
From €101 million in 2006, donations fell to €70 million in 2016, and Nuzzi quotes a document saying that the number is now below €60 million.
The other main cause the book blames for the Vatican's cash crisis is on bad management of its property portfolio. The Vatican's real estate office, Aspa, manages 2,926 properties made a loss of €22.6 million last year.
Nuzzi was a central figure in the Vatican leaks scandal that revealed alleged corruption. He published documents leaked by Pope Benedict's butler in 2012 and wrote a book in 2015 which led him being tried and acquitted by a Vatican court for publishing confidential information.