A section of the flooring at the sewage treatment plant at Taċ-Ċumnija (near Anchor Bay) collapsed earlier today, the Water Services Corporation said.
The structural damage took place in a new section where treated sewage water is further 'polished' for use in agriculture.
It said the damage developed during testing of works by a contractor.
An investigation has been launched.
In a separate statement, the government said it had appointed a number of experts to establish how the structural damage was caused.