NI Water have hit out after rubble weighing around four tonnes was removed from a sewer in Co Antrim.
The rubble was discovered in a sewer in the village of Aghalee, while stones were also pulled from a sewer on the Stewartstown Road in west Belfast.
NI Water said the rubble was most probably "irresponsibly dumped" down a manhole during the construction of a housing development and took hours to remove.
Denis McCavigan, wastewater manager at NI Water, said "rogue discharges" like these are commonly found around building sites where concrete is washed into the drainage system.
"This is typical of material getting into our sewers from new building works and developments. Combined with bricks, stones and inappropriate rags, they can become a real pain to remove and a very expensive piece of maintenance work," he said.
“This act shows not only lack of respect for our network but also for the people living in the area.
"NI Water would remind all customers that opening a manhole to discharge any material or washing substances into the drainage system is prohibited.
"By doing this, you are obstructing a working sewer which is only designed to take wastewater away from homes and businesses.
“By obstructing the sewer in this way, it reduces the pipe capacity and the volume of wastewater it is able to take; this can lead to increased blockages and out of sewer flooding in the street or into a river."
NI Water urged customers to be mindful of what they flush or put down the drain.