The city of Mississauga has been named in a lawsuit for $200 million.
Peel region, the Halton Regional Conservation Authority and the Environment Ministry are also named as defendants.
The lawsuit is because a number of residents' properties had rainwater and sewage flood their basements at least seven times since 2009.
City staff failed to inform the city of the lawsuit that was filed months ago.
They only learned of the lawsuit when a councillor met with city staff two weeks ago to discuss a consultant's report into what caused the flooding.