Uisce Eireann says Lough Owel's supply has been badly affected by the summer's warm weather.
Over 50,000 customers in Westmeath will continue to be affected by a water conservation order until October at the earliest.
The Mullingar Regional Public Water Supply has been under a hosepipe ban since May this year, with Uisce Eireann extending it for an additional month up until October 13th.
Lough Owel is the source of the supply which serves customers in Mullingar and its surrounding areas, with the semi-state body stating levels at the lake remain critically low.
Uisce Eireann says despite recent rain, record breaking summer heat meant underground springs feeding the lake have slowed down the conservation process.
Hosepipe bans also remain in place for people living in Oldcastle, Kells, and Millford.
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