Cardinal George Pell wrote an essay in the Catholic Herald entitled "The days of ripping off the Vatican are over".
In the essay he said Vatican finances were now better than expected, because of the discovery.
Cardinal Pell did not reveal the exact figure he had found during his accountancy, but said the Vatican had "substantial assets and investments."
He wrote: "Many will remember the scandals at the Vatican bank (IOR) in the early 1980s, with... the Vatican being constrained to pay $406 million (£259 million) in compensation.
"Comparative quiet then returned, until the international laws against money laundering needed to be applied within the Vatican.
"The authorities supervising the Vatican bank did not move swiftly enough, and some tens of millions of euros were frozen... It was only this November, after years of dialogue and good work, that the €23 million (£18.3 million) were released.
"Since his election Pope Francis has explicitly endorsed the programme of financial reforms, which are well under way and already past the point where it would be possible to return to the 'bad old days'.
"These reforms are designed to make all Vatican financial agencies boringly successful.
"Donors expect their gifts to be handled efficiently and honestly, so that the best returns are achieved to finance the works of the Church, especially those aimed at preaching the Gospel and helping the poor escape from poverty.
"A Church for the poor should not be poorly managed."
