SYDNEY'S water reserves could reach full capacity within three months, raising the prospect that drinking water will be flowing out to sea afterwards while households are paying millions of dollars for desalinated water.
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The surging level of Warragamba Dam - now almost 85 per cent full - will top out by the middle of May if the current weather pattern does not change.
Regardless of the rising level - which has climbed 4.9 per cent in the past month - the Kurnell desalination plant will be kept in operation until the middle of June as part of a two-year ''proving period''.
By that date, experts say, the state government will be in the ''embarrassing situation'' of watching millions of litres of ''free'' water wash away into the environment while each household pays an estimated $96 extra a year for treated seawater.
According to analysis by the Greens - and endorsed by Professor Stuart White, of the University of Technology Sydney, one of the authors of the Metropolitan Water Plan - the value of the water flowing over the spillway before the plant could be switched off would be worth $82 million.
If rainfall mirrored that of 2010, that would increase to $140 million worth of water sluicing through the drum gate at Warragamba.
Former Labor premier Morris Iemma ordered the electricity-guzzling desalination plant be built in 2007 after the dam level hit a low of 33 per cent. Since then, levels have climbed, as storms associated with the La Nina weather pattern have dumped water in the catchment region and increased the inflow rate in recent months. In the past week, inflows added more than 2 per cent to storage. At this rate, the dam will be full by May 14.
If the weather of February 2010 was repeated, the dam would be at 100 per cent within six weeks.
In 1998 - the last time Warragamba went to 100 per cent - the dam level rose by 20 per cent in 12 days. In September, The Sun-Herald revealed the dam would already be above 94 per cent if Sydney Water had not quietly stopped pumping water into Warragamba from the Tallowa Dam in the Shoalhaven. That dam is already at 100 per cent and liable to ''environmental releases''.
Professor White said: ''The government could be put in the embarrassing situation of having a desalination plant in operation under its contractual obligations and at the same time having Warragamba spilling over.''
The Minister for Finance, Greg Pearce, who is leading the privatisation of the desalination plant, said the government's hands were tied by a dud deal signed by Labor in 2007.
"Sydneysiders are locked into a two-year proving period ending on June 15 this year thanks to the contract signed in 2007 by the former Labor government,'' he said. "The deal negotiated by Labor locked us into an arrangement under which the plant had to run at full capacity for two years and we have to pay the same amount regardless.''
Sydney Water was able to negotiate a reduction in the plant's output with operator Veolia in December. Output was reduced from 250 mega-litres a day down to 90 megalitres - roughly 36 per cent of capacity.
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