The documents were found hidden in the home of the parents of an official from the Venezia Nuova consortium overseeing the construction of a €5.5 billion flood barrier to save the canal city from rising sea levels.
The papers contained a list of officials getting kickbacks of tens of thousands of euros for construction contracts with the amounts received, Venice's Il Gazzettino daily reported citing documents from the investigation.
La Nuova Venezia earlier published a police wiretap in which another consortium employee is heard telling a consultant: "Write it on different paper. Edible paper. You can swallow it if someone arrives one day. I'm not joking".
The other man was reported saying: "They came to my house in the evening and told me they were arresting everyone tomorrow. If they get me, they'll throw away the key".
Investigators documented bribes in the millions of euros which were allegedly used for illegal party financing.
Venice mayor, Giorgio Orsoni, is among 35 people arrested in connection with the allegations.
The flood barrier was started in 2003 but has been badly delayed and is scheduled for completion in 2016.