WARRAGAMBA Dam could spill over within days - for the first time in 14 years - as a massive rain band drenches New South Wales, sending drinking water worth millions of dollars down the dam wall and out to sea.

Sydney's desalination plant will still operate - despite the dam potentially being full for weeks or months - with one of NSW's most experienced water experts calling for the plant to be immediately shut down.

The weather bureau last night said between 100mm and 300mm of rain could soak a vast area of the state over the next three days. It issued a severe weather warning for flash flooding for 11 districts.

"This is perhaps the most significant rainfall event that some of these areas have seen in decades and records may be broken." the bureau's NSW regional director Barry Hanstrum said.

Couple, baby rescued as rain lashes state

The SES issued a flash flood watch for the Hawkesbury-Nepean Valley.

"We are advising people to be prepared as there is a 70 per cent chance of moderate flooding," a SES spokesman said.

Weather bureau meteorologist Julie Evans said Warragamba's huge catchment area was set to receive very heavy falls.

"Some towns in far western NSW could get their entire annual rainfall in one day," Ms Evans said.

The Sydney Catchment Authority said Warragamba was 88 per cent full.

"If we get 80mm of rain over two days in Warragamba's 9000sq km catchment area ... then the dam will spill," SCA spokeswoman Karen Smith said.

"It is looking very likely and, with heavy rain predicted for all the rivers and tributaries which supply water to the dam, it could spill over as soon as Friday. The last time it spilled was in August 1998 - and that was just a very small spill."

Ms Kelly said Warragamba was not a flood mitigation dam like Brisbane's Wivenhoe Dam and could not pre-release water ahead of the deluge.

Professor Stuart White from The University of Technology's Institute for Sustainable Futures, one the authors of the state government's Metropolitan Water Plan, yesterday said it was ridiculous Sydney households were paying an estimated $96 extra in water bills because of the desalination plant when "free water" was set to pour over Warragamba's walls.

He said the desalination plant "should be put in moth balls until the next severe drought".

"It can't be switched on and off all the time. It's my understanding re-starting the plant can cost around a million dollars each time," he said.

It is costing Sydney a lot of money and its not needed right now.

"Every megalitre of water from the desal plant costs between $700 and $800. A megalitre from Warragamba would be between $100 and $200. Last week NSW dams increased their levels by between 50,000 and 60,000 megalitres - that's something like $50 million of water in a week."

The former Labor state government's desalination plant deal included a scheme to keep it in operation for two years as a "proving period", whatever the state of Sydney's water supplies.

Last night the state government refused to reveal whether it could shut down the plant in mid-June.

"The government is considering its options within its contractual obligations," a spokesman said.

Current dam levels:



Sydney catchment

Cataract: 97190ml, 100 per cent

Cordeaux: 93640ml, 97.5 per cent

Avon: 146,700ml, 69.8 per cent

Nepean: 67,730ml, 80.6 per cent

Woronora: 71,790ml, 72.9 per cent

Warragamba: 2,027,000ml, 88.1 per cent

Prospect: 33,330ml, 93.2 per cent

Wingecarribee: 24,120ml, 62.5 per cent

Fitzroy Falls: 9950ml, 89.6 per cent

Tallowa: 7500ml, 100 per cent

Blue Mountains: 2890ml, 96.9per cent

Outside Sydney

Toonumbar: 11236ml, 102 per cent

Pindari: 311695ml, 100 per cent

Copeton: 1,341,322ml, 99 per cent

Split Rock: 325960ml, 82 per cent

Keepit: 417693ml, 98 per cent

Chaffey: 62370ml, 101 per cent

Glenbawn: 750715ml, 100 per cent

Glennies Creek: 280878ml, 99 per cent

Lostock: 20329ml, 101 per cent

Burrendong: 988104ml, 83 per cent

Oberon: 28897ml, 64 per cent

Carcoar: 31443ml, 87 per cent

Wyangala: 942601ml, 77 per cent

Cargelligo: 35591ml, 97 per cent

Burrinjunk: 756225ml, 73 per cent

Blowering: 1317828ml, 80 per cent

Hume: 1906023ml, 63 per cent

Dartmouth: 2968762ml, 77 per cent

Brogo: 9084ml, 101 per cent

Lake Cawndilla: 571551ml, 89 per cent

Lake Wetherell/Tandure: 103684ml, 53 per cent