The Waikato River won't suffer any ill-effects if Auckland gets to take twice as much water from it, an expert says.
But the fast-growing city can't rely on it forever says Bethanna Jackson, an associate professor in physical geography at Victoria University.
Auckland is struggling to emerge from the worst drought it's had since the early 1990s. Even with the past week's rainfall, its storage lakes are still only 55 percent full - well below the usual 80 percent at this time of year.
Waikato officials and iwi initially balked at the city's request to draw more water from the river, Waikato Regional Council chair Russ Rimmington fearing it would suffer the same fate as Australia's Darling River, which barely flowed at all at times in 2019.
An emergency summit earlier this week saw various parties iron out their differences, allowing plans for New Zealand's biggest city to take more from the Waikato while it develops more permanent, long-term solutions such as recycling wastewater.
But fears Auckland's water demands will ruin the river are probably overblown, says Dr Jackson - in fact, the city's investment might actually improve the Waikato's health.
The first reason is the location - Auckland takes its water from a site near Tuakau, which is close to the river's mouth. Taking water from there will have no impact on levels upstream, Dr Jackson told Magic Talk on Friday.
"It would be very unusual for that to happen unless the river was extremely flat and the river was also quite small."
The Waikato River's mouth is also in an area where there are strong tides, and these have much bigger influence on water levels and flow than the amount Auckland takes out. At the moment Auckland takes about 0.5 percent of the water that goes past.
"Relative to the tidal influence, that's not particularly important."
The Waikato also has a very consistent amount of water flowing at any one time, never getting low enough that Auckland's take would cause issues.
"There are a wide variety of rivers around New Zealand and the world where some are very flashy - when you get a rainfall event most of the water ends up in the river pretty quickly; the Waikato, partly because of Tongariro and Lake Taupo, has a much more sustained, steady flow."
There is also a steady flow of water coming in through the ground. The Darling River in Australia on the other hand is a very arid system, Dr Jackson said, and is "very flashy" - when it rains, it flows; when it doesn't, it doesn't.
It's hard to measure the constant flow at Tuakau because the tidal influence is so strong, said Dr Jackson, but further upstream at Huntly - home to one of the country's biggest power stations - it's about 420 cubic metres per second.
"It's going to be a lot more than that once you actually get down to the point where they're actually extracting the water. It's a fairly sustained river."
Since the boosted extraction won't have any ill-effects itself, the overall effect of Auckland's new investment might actually improve the river's condition. A trust fund of "several million" dollars has been set up to look after the river's health, that otherwise wouldn't exist.
Auckland Mayor Phil Goff on Thursday said long-term, he wants to build a water recycling system like many overseas metropolises have.
"[In Singapore] every drop of water you've ever drunk has been through somebody's kidneys a dozen times. Probably 100 times," he told The AM Show. "It doesn't sound great, but the time we have recycled that water and treated it, it's as pure as the water that falls from the sky.
"People screw up their nose and I was told it's just not appropriate or it's culturally offensive - no, we've got to do that. That is the guaranteed source of water as our city hits 2 million people - we simply recycle the water."
Dr Jackson said it's essential Auckland find another source of water, whatever it is, as even a river as mighty as the Waikato has its limits.
"It's not a source you can take from for all-time - if Auckland grows to 10 times its size and you're talking about 10 percent of water or even more, then at that point you've got a problem."