The public should not resort to 'panic storing' of water to cope with the massive disruption of water supply in Kuala Lumpur and Selangor.

Ministry of Energy, Green Technology and Water secretary-general, Datuk Seri Zaini Ujang said to ensure that water supply could be restored as scheduled, the users should not store water more than needed.

"For example, a household that normally uses one cubic metre of water should not be storing three cubic metres of water, this is 'panic storing'. The water is available (during disruption) but it is very limited.

"As such, the users do not have to keep too much water. Do not store water more than necessary," he told a press conference on the water supply disruption at the ministry, here today.

He said the water supply disruption in Petaling Jaya, Shah Alam, Gombak, Kuala Langat, Hulu Selangor, Kuala Selangor and Kuala Lumpur is expected to be fully restored on Friday (Dec 23).

"When 'storing panic' happens, the system will become empty. This means we need more time to fill up the pipes, the tanks and the reservoirs." Zaini said the decommissioning of water treatment plants means that areas at the end of the supply chain and on high ground will have longer disruption.

"This is because adequate water is needed so that the distribution system can operate as usual in low reserve margin of two to three per cent."

The water supply disruption is due to maintenance of Tenaga Nasional Berhad's (TNB) sub-station at Bukit Badong, Kuala Selangor.

Syarikat Pengurusan Air Selangor Sdn Bhd provides 64 water tankers for supply to the users, especially for festive occasions and emergencies.

Zaini said Dec 19 was chosen for maintenance work after considering factors such as decommissioning of Langat Water Treatment Plant on Dec 6 and completion of school examinations.

"Another factor is the low usage of water per capita at the end of the year compared to the other months.

"Based on data monitoring of Bukit Badong TNB sub-station, the ministry decided that maintenance work must be done immediately to reduce the risk of serious power failure," he said.

The water supply disruption is expected to affect some 3.8 million users in 814 areas in Kuala Lumpur and Selangor.