As at 12.30pm, the reservoirs have enough water to last about 12 hours' average use.
It is not known when the problem will be fixed.
"The pipes are at least six metres underground so it will take us some time to expose the leak and carry out the necessary repairs," Mr Laidlow said.
He said it appears repair teams will be working into the night to remedy the problem, and urged residents to conserve water until further notice.
"For example, don't leave taps running unnecessarily for example when brushing your teeth, have very short showers, avoid using the dishwasher and let those dishes stack up on the bench if you can, don't flush the toilet every single time, don't water indoor plants or your garden, and please don't do any unnecessary cleaning such as cars or windows."