If climate and population trends persist, Aucklanders could one day be drinking treated sewage - if they can get over the "yuck" factor, that is.

For Watercare chief executive Raveen Jaduram it is less a case of if, but when, and he thinks the city needs to start "mature" conversations about the potentially taboo topic right now.

Amid the worst drought on record, Auckland Council this week signed off mandatory restrictions for the first time since 1994, when the region's supply dams dropped as low as 29 per cent.

Then, there was no emergency drought back-up, and in response the council sought consent to supplement its dams by taking 150 million litres a day (MLD) from the Waikato River.

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Watercare chief executive Raveen Jaduram and Mayor Phil Goff ahead of the meeting to introduce mandatory water restrictions. Photo / Michael Craig
Watercare chief executive Raveen Jaduram and Mayor Phil Goff ahead of the meeting to introduce mandatory water restrictions. Photo / Michael Craig

But despite Watercare taking as close to its full take from the Waikato as possible since January, the dams this year have still dropped to 45 per cent in the driest January to April period on record, with 77 consecutive days spent in drought or severe drought.

The region has experienced drought in three of the past four years, and six in the past 10, according to Niwa.

Meanwhile, the city grows - over 50 per cent since 1994 - and with climate change bringing increasingly dry summers, Auckland's thirst increases as the supply shrinks.

Water storage in Auckland has been below historical average since March 2019. Image / Watercare
Water storage in Auckland has been below historical average since March 2019. Image / Watercare

When full, the dams in the Hunua and Waitākere ranges - the region's rainmakers - contain about eight months' water for the city's 1.6 million inhabitants, generally providing about two-thirds of daily usage, which averages about 450MLD a day.

Smaller aquifers and bores also assist, but without any major underground sources and no desire to further dam the ranges, the council applied to take a further 200MLD from the Waikato River in 2013.

Under the Resource Management Act, such applications are "first-in, first-served", meaning the council sat at around 200 in line.

It has since risen to about 94 - but this week Auckland mayor Phil Goff called that process "unacceptable", and revealed he'd written to Environment Minister David Parker that the system needed to be changed.

Jaduram estimates based on population growth and climate projections the city will be in need of that extra supply by about 2025, perhaps a little later.

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That supply will suffice for at least another decade - but beyond then Jaduram says Auckland needs to start looking at "circular" options, such as reusing wastewater.

Extremely low dam storage levels in the Waitākere Ranges. Photo / Watercare
Extremely low dam storage levels in the Waitākere Ranges. Photo / Watercare

Desalination was also an option, but it was very energy intensive and expensive.

"Where we are, we can't really increase storage, and that is still dependent on rain.

"Treating wastewater is the most environmentally-friendly and preferable option. It is a closed loop system, though it presents some societal challenges - the 'yuck factor'."

It is used in about 35 cities around the world, and has been in parts of the United States since the 1960s. Even Thames Water in London is looking into it.

In Perth - which has seen dramatic decreases in rainfall due to climate change - about 14 billion litres of recycled water is added to the city's aquifers every year, with work to double capacity due for completion next year.

In practice, it would see the water coming out of the Māngere wastewater treatment plant given further treatment, passed through a range of filters - such as aquifers or wetlands - before another round of treatment to ensure it was at a drinkable standard.

Rainfall this year in the Hunua Ranges. Image / Watercare
Rainfall this year in the Hunua Ranges. Image / Watercare

"It is normal in many places around the world, Singapore is leading the way," Jaduram said.

"If there is nothing else, what do you do?"

Legislation would need to change to allow it here, as would society's views, he said.

Rainfall this year in the Waitākere Ranges. Image / Watercare
Rainfall this year in the Waitākere Ranges. Image / Watercare

The town of Toowoomba in central Queensland is a case in point, where the community wholeheartedly rejected the idea, despite it being commonplace in other parts of Australia.

"But water is going to become a bigger issue. There is a view that New Zealand has a lot of water, but areas like Auckland will be seeing less rain with climate change.

"It's not the distant future - it is happening now, and we need to be having these mature conversations."

Other ideas included installing rain water tanks on all new builds and promoting grey-water systems, to both store water along with reducing pressures on the stormwater network.

Auckland's metropolitan water supply network. Image / Watercare
Auckland's metropolitan water supply network. Image / Watercare

Jaduram said they would help, but not in times of drought.

"This summer we've seen all of those on tank supply needing to be refilled from the city supply. The problem is they also run out when it doesn't rain."

The idea could also run into opposition from tangata whenua.

University of Auckland freshwater and Māori studies expert Dan Hikuroa said Māori tikanga opposed mixing of waimate - water devoid of life - and waiora - fresh, healthy water.

But he didn't see treated wastewater as "waimate".

"It is degraded, but it is not dead water."

Hikuroa - of Ngāti Maniapoto, Tainui and Te Arawa - is part of Watercare's environmental advisory team, and said the ability to recycle wastewater came down to ensuring the mauri, or life-supporting capacity, was restored.

"That is done by passing it through Papatūānuku, the earth."

The idea stems from springs, where water had been naturally filtered through the earth, and which historically Māori held in incredibly high regard for its mauri.

"There is nothing more pure and life-supporting than that. So pragmatically speaking, we'd want it to be scientifically treated, and then passed through a sort of earth-based treatment.

"We'd probably be looking at it being used in more industrial processes first though, that'd be an easier hurdle."

The Lower Nihotupu Dam in the Waitākere Ranges at less than 50 per cent of storage capacity at May 3. Photo / Watercare
The Lower Nihotupu Dam in the Waitākere Ranges at less than 50 per cent of storage capacity at May 3. Photo / Watercare

In the meantime, Hikuroa said people needed to treat water with more care.

"We need to treat the whole system as taonga, look to conserve and collect as much as we can."

This was consistent with Watercare's messaging, with Jaduram saying conservation would delay needing to increase supply.

He acknowledged this was already relatively low with residents averaging about 157 litres a day, compared to 263 nationally, but every bit counted.

Other measures to investigate were signals and tariffs, such as increasing the price in summer to discourage excessive use.

"Increasing supply is costly, so we really need to look at being more conscious with what we have and how we use it," Jaduram said.

"We can't just expect to keep taking as much as we want from the environment, plus we can't afford to."

Infrastructure group Water NZ acting chief executive John Mackie said it would be hard "selling the concept" of reusing wastewater.

He suggested storage options should be looked at more closely including potentially expanding dams and reservoir, although this was limited in Auckland by land supply.

Another option was to reticulate treated wastewater to consumers for non-potable purposes such as irrigation or industrial processes.

"It is a sad irony that parts of New Zealand have an abundance of water while other regions are in drought, but there are other ways to deal with that before we would need to consider using wastewater for drinking water purposes."

Auckland's water shortage

• Stage 1 water restrictions will be in force from May 16, and prohibit the residential use of outdoor hoses and water-blasters unless for a health, safety, emergency or biosecurity reason.

• Under stage 1 commercial car washes are also banned unless they use recycled water; and watering of sports fields, plants or paddocks is restricted to those with an irrigation system fitted with soil moisture or rain sensors.

• Watercare further advises residents to keep showers short - four minutes or less, and only run the dishwasher or washing machine when they're full. No restrictions apply to hygiene measures, and people should continue regularly washing their hands consistent with Covid-19 messaging.