Auckland has been given the all-clear to draw 150 million more litres of water a day from the Waikato River.
This means Watercare now has consent for 300 million litres of water a day from the river all-year round, doubling the volume for the region.
A board of inquiry appointed by the Minister for the Environment gave the green light to Watercare's resource consent application on Friday.
The company had wanted a 35-year-limit for the allowance, but the board decided on an operational consent for 20 years.
Watercare chief executive Jon Lamonte said the application process had taken many years, and was thankful for those involved.
"Securing this consent for the people of Auckland has required a marathon effort. I'm truly thankful to everyone involved, from the central and local government officers to community groups to Waikato Tainui and other iwi representatives," Lamonte said.
"At times the conversations have been challenging and confronting but I think we've emerged from this process stronger and with a desire to work collaboratively for the good of the awa."
He said work on building a new permanent water treatment plant was under way, which would be timed to meet Tāmaki Makaurau's population growth.
In 2020, Watercare had conditional consent to take an additional 100 litres a day, subject to awa's level, during winter after a severe drought hit Auckland.
That summer the region experienced 40 days with no rain, and dam levels were about 60 per cent, or 30 billion litres short of what's normal for that time of year.
In July last year, a $145 million water treatment plant in Tūākau was also opened - as part of a $224m package to help drought recovery approved by Auckland Council, which provided up to 50 million litres a day from the Waikato River.
"Waikato Regional Council may, every five years, review these consents to consider a range of matters including the appropriateness of any take rate or take volume," the board said.
Watercare was also required to invite tāngata whenua and establish an executive committee to help investigate how the volume taken, and Auckland's reliance on the wai of the Waikato River, could be reduced.
Auckland mayor Phil Goff said the decision would help ensure the resilience of Auckland's water supply.
"With population growth and the impact of climate change meaning more frequent and extreme drought, we have to be able to guarantee the reliability and resilience of the city's water supply," Goff said.
"Drawing water from the Waikato River before it is discharged into the sea does not affect the wellbeing of the river nor other applicants for water use, dropping the level of the river by just a few centimetres where it runs around six metres deep."