Applicability of GST on Water Sewage Charges

Janta Land Promoters Pct. Ltd. has constructed Falcon View Apartments in Mohali. As other elements of the project are still under contraction, the builder is providing the maintenance. While GST on maintenance is applicable, he is also charging GST on water and sewerage charges. Is this justified? The Chandigarh Municipal Corporation does not charge GST on water and sewerage.