Before the 2007 election, premier Morris Iemma promised it would be built when dam levels hit 33 per cent.

The current government is now railing about a “stupid contract” that means the plant will stay in operation for two years regardless of the state of Sydney‘s water supply.

But an engineer who was involved in designing the Kurnell plant said this was playing politics over sense.

“It was an insurance policy,” Brett Miller from the UNSW Water Research centre said.

“Sydney at the time was in a drought. A large proportion of our water was coming from the Shoalhaven. I really don’t see that you could have made any different decision.”

Mr Miller said Sydney runs through a wet/dry cycle – meaning it was likely that in a few years pouring rain would give way to drought.

Mr Iemma said the then government decided to build the plant based on advice Sydney’s water supply was disappearing at an alarming rate.

“The advice was that (dam levels) were going to hit 27 per cent in a very short period of time. We just needed to provide certainty,” he said.

The 2006 metropolitan water plan, which supported the decision to build the plant, noted that the probability of dam levels reaching 30 per cent was “very low” but building the plant was “vital to ensure that Sydney’s water needs can still be met should this situation occur”.

The desalination plant announced on Wednesday that, following heavy rain, it would reduce the volume of water it supplied to 45 megalitres a day.

Finance Minister Greg Pearce said he welcomed the decision to cut the volume of water produced. The two-year “proving period” for the plant expires on June 15, and Mr Pearce said it was not viable to shut the plant before then.