A detailed business case has recommended two projects as the best solutions to ensuring water security for the Darwin region amid growing concerns over water supply.
The Adelaide River Off-Stream Water Storage (AROWS) and the Manton Dam Return to Service (RTS) projects are expected to supply 67 billion litres of water a year at full capacity for the Darwin region, with construction expected to cost $1.45 billion.
The AROWS project has long been promoted as a solution to Darwin's water security needs, by diverting floodwater from the Adelaide River — about 80 kilometres south of Darwin — into an off-stream basin during the wet season.
Now, both projects have gained the support of a new business case conducted by professional services firm PriceWaterhouseCoopers.
Speaking to reporters on Tuesday, Power and Water Corporation chief executive Djuna Pollard said "traditionally, we've had essentially one main water source".
"It's very much time that we diversify our water supply so that we can provide a secure water source moving forward," she said.
NT Water Security Minister Eva Lawler said the projects would take a substantial amount of work to get up and running.
"But the first and foremost is continuing the work with the Australian government around funding this project," she said.
A solution for the next 50 years
Most of Darwin's drinking water comes from the Darwin River Dam, built in 1972.
Before that, it was Manton Dam, built in the early 1940s.
But an expanded population, a run of below-average wet seasons in recent years and the growing effects of climate change have put more and more pressure on water levels at the Darwin River facility.
Two years ago, water levels plunged to their lowest since 2006, falling to 52 per cent of its maximum capacity.
Demand for water at the dam is already slightly above capacity.
By 2050, it's expected an additional 11 million litres will be needed per year to meet population growth.
The Manton Dam RTS project is proposed as a short-term solution to that problem, with development work expected to take between three and four years.
Ms Lawler said the government was keen to start work as soon as possible in 2022.
"The work around Manton Dam is that environmental impact work, it is around the design work that needs to be done … there's a new pipeline that'll need to be installed as well," she said.
The AROWS project is expected to take between seven to 10 years, with works to include a water allocation plan for Adelaide River, environmental impact assessments, and consultation with landholders.
"This project will be the biggest project undertaken by the Northern Territory government, by Power and Water, since the rail that was built quite a number of years ago," Ms Lawler said.
'Not damming the Adelaide River'
Although Manton Dam hasn't been used for drinking water in decades, it's now popular for swimming, water skiing, and fishing, as one of the few (mostly) crocodile-free water sources in the NT.
Ms Lawler said recreational use would continue for now with a decision to be made this year, but said it could continue to be used for both.
"There would be a cost around that, but there is an infrastructure solution around treating the water to make sure Manton Dam can continue to be used for recreational purposes," she said.
She also reassured Territorians the AROWS project would not be damming the Adelaide River.
The Amateur Fishermen's Association chief executive David Ciaravolo told the NT Country Hour the off-river storage system was preferable to damming the Adelaide River but said the design would need to be "well-balanced."
"We have to make sure that the offtake that comes from the Adelaide River comes at the right time and only in the quantities that are environmentally appropriate," Mr Ciaravolo said.
"And of course, the other thing we need to be sure of is that when they're taking that water, that there is appropriate screening of those pumps.
"There are estimates that there are millions of millions of fish getting killed in the Murray-Darling system every year due to water pumping and we need to make sure that doesn't happen here."
