SDLP MLA Nichola Mallon has also urged NI Water to put a more robust system in place to prevent tampering with hydrants.
Ligoniel and New Lodge are among the areas that have been affected.
NI Water said some staff had come under threat as they tried to close hydrants.
It stressed that fire hydrant abuse was illegal and appealed to community and political representatives "to continue to support us in tackling this issue".
"There have also been incidents where NI Water staff have come under threat from local youths when they try to close the hydrant down, having to return late into the night when the crowds have gone," NI Water said.
"This is totally unacceptable behaviour toward staff who are trying their best to do their job and protect the water supply to customers.
"The whole community needs to help us put a stop to this behaviour before the unthinkable scenario occurs where a fire breaks out and there is no water for the NI Fire & Rescue Service to deal with the fire."
Ms Mallon said the disruption began on Sunday.
"It is particularly hard for parents with young children when there is no access to water to bath them and in this hot weather drinking water - for the likes of elderly people it is also a hardship," she said.
"Some young people might think this is harmless fun, it is not, it is causing serious disruption to people's lives.
"For Northern Ireland Water there must be a more robust way of securing the hydrants to prevent this abuse, surely in this day and age there must be a way of securing them so they cannot be tampered with."
NI Water said it had used "vandal-proof" hydrant lids in an attempt to stop the hydrants being vandalised, but said that "if someone has the time and the resources, they can eventually break these open".
Young people have been warned that vandalism of fire hydrants is not "harmless fun", with disruption being caused to water supplies over four days in north Belfast.