The decision to publish the informnation has been attacked by consumer groups and some politicians.
ADOC, an Italian consumer group, said: "It's a clear violation of privacy law. A danger for an increase in crime and violence as the data are an irresistible source for criminals."
"It's all about transparency and democracy. I don't see the problem"
The group said the fact that tax return forms do not include a request for consent to the data being published means their appearance on the internet was clearly illegal.
However, Corriere della Sera, the Italian daily newspaper, said the website was "a delicious opportunity to find out with a click how much your neighbour or colleague or, for gossip fans, celebrities earn".
Tax evasion is rife in Italy and both Silvio Berlusconi, the country's incoming prime minister, and the man he succeeds, Romano Prodi, have promised that combatting tax cheats would help plug a hole in the budget.
However, many Italians resented the crackdown under Prodi, and Berlusconi himself said during the election he would stop the "atmosphere of threat and terror" that had been imposed.
Vincenzo Visco, Prodi's tax minister, said the website was meant to go live in January, but was delayed until now to avoid clashing with the campaign for the April 13-14 election.
Visco told Corriere della Sera's website: "It's all about transparency and democracy. I don't see the problem."
Guido Crosetto, a member of Berlusconi's party, said the move was the last spiteful measure of the defeated centre left before it hands over power next week.
He said: "It's a very strange thing to do on the last day before clearing off. Taxpayers need to pay less tax, not to know how much all the other Italians are paying."