Acea Ato 5 reports

From January, the previous 5-year period is reduced to 2 if the delay is imputable to the provider.
The minimum billing period is set to one month.

Starting on 1 January 2019, the prescription period for water usage was reduced from 5 to 2 years, for domestic use, small businesses and professionals. Now, in the event of significant billing delays imputable to the Provider, users can object and pay only the amounts due for water usage within the past 2 years.
The change, already in effect for electricity and gas bills, was introduced in the water industry with Financial Law 2018 but has come into effect only this year.
The two-year prescription applies to bills that expire after 1 January 2020.
Under the new rules, water service providers can issue a separate bill including only the water usage dating back more than 2 years, or, alternatively, they may highlight those amounts in the same bill that contains more recent usage, as long as the distinction is clear and easily understandable.
When the billing delay is exclusively imputable to the Provider, they are obliged to inform users of the option to object to the payment of the amounts dating back more than two years and provide an easy format to communicate their intention not to pay.
In fact, it should be noted that the prescription is not automatic: users have to request its application. However, Providers can autonomously choose to renounce to their right to the amounts dating back more than 2 years and adequately inform final users of their decision.
On the other hand, if the user is presumed responsible for the billing delay, the Provider must provide the reasons why they attribute the delay to the user and inform them of the option of presenting a complaint and the procedures to follow.
Arera, the Authority for the Regulation of Energy, Networks and Environment, has regulated the matter with decision 547/2019/R/idr, which with the intent of avoiding excessively frequent bills, sets the minimum billing period at one month. The same decision also aims to encourage the improvement of the service and contractual relations by implementing a mechanism of incentives and penalties to improve quality standards and the contractual relation between providers and users.

Moratorium for unregistered users
Acea Ato 5 reminds users that following an agreement signed with the Area Provider and the Consumer Protection Body, a moratorium for unregistered users of the integrated water service has been introduced – in the interest of all users – with the aim of encouraging contract stipulation and reducing water tariffs.
As an incentive to regularize the position of unregistered users, the agreement stipulates that anyone who voluntarily regularizes their position will not have to pay for prior water usage, as follows:
anyone who regularizes their position by 31 March will pay nothing more than the activation fee (approximately €100 for domestic supply); instead, those who regularize their position by 30 June, in addition to the activation fee, will also have to pay a third of the average yearly consumption for their type of supply; finally, anyone who regularizes their position by 31 December 2020, in addition to the activation fee, will also have to pay a third of the average consumption.