Lower Hunter residents could be using desalinated water sooner rather than later unless there is a significant shift in rainfall patterns in coming months.
Under present planning, an emergency desalination plant proposed to be built next to the Belmont waste water treatment plant would be turned on when water storages reach 35 per cent.
Lower Hunter water storages reached 65 per cent for the first time in 21 years this week.
It is estimated Level 1 water restrictions will be introduced in mid-September.
Hunter Water's residential customers are presently consuming more than 107 kilolitres of water a day. Combined with low rainfall, storage levels could reach 35 per cent in the first half of 2020.
The last time storages were this low was in the drought of the early 1980s.
Former Hunter Water managing director Jim Bentley, who is now the state's top water bureaucrat, said choosing the right time to flick the desalination switch was a complex balancing act.
However, he agreed it was essential not to leave the decision too late.
"We need to bear in mind (water) resilience isn't created just by building a piece of kit. Resilience is created by being adaptable and flexible but you have got to be adaptable and flexible without risking not having enough water," he said.
"What the right level (trigger) is to build a piece of infrastructure that you are only going to need if you are in a drought is complex. If you invest in it and the drought ends you have it sitting there having spent all that money and energy to build the plant.
"We are not going to take risks, we are going to make sure we have done it early enough. It could be that we need to review if 35 per cent is too late.
Hunter Water applied to the Department of Planning in late 2017 for approval to build the "off the shelf" desalination plant that would be capable of supplying up to 15 million litres of water per day.
The project is in the final stages of the approval process.
About 1 per cent of the world's population is already dependent on 20,000 desalination plants.
Demand is expected to soar as water scarcity worsens with climate change and rising use.
Sydney's desalination plant was re-activated in 2018 and is presently producing about 850 million litres a week.
At full capacity, the plant will supply 250 million litres of drinking water daily, or about 15 per cent of Sydney's needs.
Despite concerns about the environmental impacts of desalination, a 2018 study concluded the effects on the surrounding marine ecosystem were not as great as first thought.
The report, published in Water Research, found the $2 billion plant had a negligible effect on the offshore ecosystems during the two years it was operating before it was mothballed in mid-2012.
"We're fortunate and relieved that the ecological impacts are minor compared to what people thought might have happened with this development," Emma Johnston, a professor of marine ecology at the University of NSW told the Sydney Morning Herald.
At 100 metres from the outlet points, water salinity was barely distinguishable from the ambient conditions because of the way the brine was successfully spread in the water column, Professor Johnston said.
The Australian Water Association estimates the cost of supplying desalinated water varies widely, from $1 to $4 per kilolitre, depending on location and how much is used.
"The issue of water pricing leads on to the question of what happened to the alternative strategies - recycling and demand management - that cities pursued before desalination became the favoured approach?," Ian Wright, a senior lecturer in environmental science at Western Sydney University wrote in the Conversation.
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