According to the 2017 Metropolitan Water Plan, WaterNSW was to transfer water from the Tallowa Dam on the Shoalhaven River when Sydney Water levels dropped to 75 per cent full.

However, those transfers haven't happened because Tallowa's level itself was already too low when that point was crossed, WaterNSW said.

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Larger supplies, though, were expected to come from the desal plant, which at full bore can meet about 15 per cent of Sydney's water use.

However, a drawn-out spat between the privately owned plant and insurers - after the tornado tore through the control room at the Kurnell site three years ago - has meant it can't even be turned on until this December.

"The testing phase is due to be completed by December 13," a spokesman for the plant said.

Drinking water would then take "four to five months" from the restart to be produced, and full production would only be reached "by month seven and eight", he said.

Nosedive

"If there's a hiccup or two in the plan, we do need to reconsider bringing the [stage one] water restrictions sooner," Stuart Khan, a professor at the University of NSW's School of Civil & Environmental Engineering, said.

"The nosedive [in water storage levels] is even steeper than going into the Millennium Drought."

Utilities Minister Don Harwin said "Greater Sydney’s water supply is robust and secure and we are confident that, despite the dry conditions, no homes or businesses will go without water".

Storages held enough water to supply the city’s population "for another two years - even without any rain at all over that period," he said, adding that authorities "have planned for drought conditions".

Determining how fast dam levels will drop hinges on water use, catchment rainfall and evaporation losses. At current rates, that 50 per cent level for stage 1 restrictions would be reached in about six months but at 0.8 percentage-point rate, it would be hit by early January - both before the desal plant will produce water.

'Chronically underestimated'

Both Labor and the Greens blasted Mr Harwin for failing to act sooner.

Chris Minns, the Labor water spokesman, pointed to comments Mr Harwin made a year ago today when dam levels were much higher.

"If the dams weren't full, we might need to do [the plant repairs] faster, but we don't need to, so it's good," Mr Harwin told Seven News.

Last September, Mr Harwin told state parliament it was unlikely the 60 per cent trigger "will be reached in the next five years".

The minister "has chronically underestimated the threat and risks of drought", Mr Minns said. "Labor is demanding clarity on the water restrictions timetable."

Justin Field, the Greens water spokesman, said "It makes sense to bring in stage 1 water restrictions earlier to reduce the likelihood of more extreme restrictions down the track".

He said the existence of the desal plant "seems to have created a false sense of security that has meant government and Sydney Water have taken their eye off the ball when it comes to water efficiency".

"The result is even more cost on the consumer in the long term and potentially crippling water restrictions in the future."

'No cause for concern'

WaterNSW Chief executive David Harris said storages had enough water to ensure supplies until at least mid-2020 "even under the continuing drought".

“There is cause for the precautionary planning that is taking place by the NSW Government,
WaterNSW and Sydney Water, but there is certainly no cause for concern,” Mr Harris said.

“We have about two years’ supply banked in our dams, an adaptable network with the ability for
internal transfers to tackle pressure points, and a dependable, well-tested plan to guarantee supply," he said.

A WaterNSW spokesman said transfers from Tallowa Dam had not been possible earlier due to major upgrade works being undertaken on the Burrawang Pump Station. The scheme was returned to service today.

"WaterNSW can now commence Shoalhaven transfers when inflows to the system occur," he said.

Fairfax Media approached Sydney Water for comment.