The water level at Linggiu Reservoir in Johor, a source of water for Singapore, has fallen to below 50 per cent owing to the ongoing dry weather.
National water agency PUB said yesterday that water levels in the reservoir stood at 72 per cent capacity at the start of the year.
The water level last dipped below 50 per cent in 2015 to reach a historic low of 20 per cent in 2016.
"It has not fully recovered to the healthy level of 80-90 per cent, which had been the case for almost 20 years when Linggiu Reservoir began operations in 1995," it said.
Singapore built Linggiu Reservoir at a cost of more than $300 million to enable the abstraction, or extraction, of water at PUB's Johor River Waterworks (JRWW).
PUB said Linggiu has been slow to recover because more water is being drawn from the Johor River than is sustainable. "Malaysia has built water plants upstream of the JRWW, which have further added to the abstraction of water from the Johor River," it said.
The situation is exacerbated during dry spells, as PUB needs to discharge more water from the reservoir to support water abstraction.
"In the event of a prolonged drought, a depleted Linggiu Reservoir will compromise Singapore's right to abstract our full 250 million gallons per day (mgd) entitlement of water under the 1962 Water Agreement," PUB said.
In April, Prime Minister Lee Hsien Loong said Singapore and Malaysia had an interest to work together to ensure a sustainable water supply for both sides.
This includes the identification of appropriate and timely measures, including schemes, to increase the yield of Johor River, the PUB statement said.
On the ground, PUB said it has enjoyed "longstanding cooperation" with its Johor counterpart.
Recently, the Johor state water agency made a request to PUB for additional supply of treated water, as a result of disruption in production at Johor's water plant in Skudai.
PUB readily acceded to the request and had done so from Sept 23 to Sept 27, PUB said.
"Under the 1962 Water Agreement, Singapore is required to supply Johor with 5mgd per day of treated water. In practice, PUB has been supplying 16mgd of treated water to Johor at their request," said PUB. "This latest request for an additional 6mgd of treated water is on top of the 16mgd that Singapore already supplies to Johor."
It added: "Johor made similar requests this year in January and August. Last year, Singapore supplied additional water in excess of the usual 16mgd for 20 days."
PUB said it supplied all the additional treated water at the same price as under the 1962 Water Agreement - 50 sen per 1,000 gallons - which was a fraction of the cost of treating the water.
"This has been done without prejudice to our rights under the 1962 Water Agreement," PUB said.
The agency said that, with climate change, Singapore will experience more extreme weather patterns, and prolonged dry spells will affect its water supply.