Water flows into the Sydney drinking water catchment are at a record low and less than half than they were during the millennium drought last decade, prompting more concern about the city’s water security.

Previously the lowest inflows into the catchment had been 136 gigalitres in 1944. In 2004, during the height of Sydney’s last water shortage during the millennium drought they fell to 234 gigalitres.

But on Friday, at a budget estimates hearing, chief executive of New South Wales water David Harriss said that if inflows “continue along the path they’ve been going in the last couple of months” this year’s total amount into the catchment would be only 83 gigalitres.

NSW Greens MP Justin Field labelled Harriss’s evidence “shocking”.

“We’ve had drought and water security issues in Sydney in the past but this is unprecedented, he said.

“I don’t think the government has come to terms with how serious these water inflow shortages are. We’ve taken our water security for granted because of the desalination plant [and] it’s led to a perfect storm that could leave Sydney residents racing extreme water shortages over the next year or so.”

It comes as further scrutiny is placed on Sydney’s long-dormant desalination plant at Kurnell.

The plant has been in “water security mode” since 2012 when it was sold by the current Liberal government on a 50-year lease to a private consortium split 50-50 between Hastings Funds Management and the Ontario Teachers’ Pension Plan, based in Canada.

In that time the plant has been in a deep state of preservation, with sea inlets capped and the majority of the plant dried out.

But at the same time the NSW government has paid out hundreds of millions of dollars a year on what’s called an “availability charge”.

When up and running, the cost of the plant to Sydney Water would be $237.4 million per year year, or $129 per household.

But while its mothballed, Sydney Water still pays out hundreds of millions of dollars to cover the plant’s future availability.

Until last year that availability charge was costing taxpayers about $190 million per year.

After the Independent Pricing and Regulatory Tribunal reorganised the fee in 2017, Sydney Water says it now pays about $175 million to keep the plant ready to switch on.

The value for money of that fee is now under question.

Under the government’s Sydney metropolitan water plan, the desalination plant is supposed to be switched on when the city’s dam levels hit 60%.

That day is fast approaching. Currently the dam storage level is at about 65%, and has been falling by about 0.7% each week.

The utilities minister Don Harwin has maintained that the plant – which was damaged by storms in 2015 – will be ready to be switched on by December.

But officials at Water NSW have told Guardian Australia that if the inflows don’t improve its more likely the trigger will be hit in November.

Either way, this week the Guardian revealed that it could be a year before the desalination plant is at full operational capacity.

Stuart Khan, an expert in water security from the University of NSW, said the private operators “should be held” to the agreement.

“We’ve signed up to a financial agreement that involves NSW water customers forking out a lot of money. If the terms of the agreement is that the plant going to be maintained in a condition where it’s ready to go when we need then we need to make sure the plant is held to that arrangement,” he said.

“It would be like paying for house insurance and when your home burns down the insurer says well we might build you a house in six months.

“We’ve been keeping up our side of the bargain and they need to too.”

Khan has long been critical of Sydney’s approach to water security, and says a more mature conversation about the use of other sources such as recycled water is necessary.

“In the short term we’ve seen our water supply system is very effective in getting through serious droughts like the millennium drought,” he said.

“But there are two major factors in the future that will change that. The first is climate change, which I see as an uncertain change – we don’t know what’s going to happen. But the other one is population growth. We’re going to see millions of extra people in the Sydney basin over the next 50 years, double the current population of Sydney, and that’s obviously going to have a big impact on water security and water demand.”