Wellington City Council is the latest lower North Island council to join a band of local authorities planning to reform the region's water services.
The council voted on Thursday to work with others in the region on a new water delivery model.
Under the government's Local Water Done Well policy, councils will be required to produce water service delivery plans that meet regulatory and investment requirements by mid-2025.
Wellington Mayor Tory Whanau said it was crucial every council in the region worked together to "provide resilient water networks and deal with population growth".
"Councils around the country are facing similar challenges. That is why we need to work towards a new model to make funding and delivery of water infrastructure more sustainable and efficient in the long run," Whanau said.
Hutt City Council and Upper Hutt City Council committed to the unified approach last week. Other councils in the region were still considering whether to sign the joint Memorandum of Understanding.
The agreement will see councils enter a non-binding partnership.
One elected member from each council would sit on an advisory oversight group for the joint water service delivery plan process, alongside iwi/Māori representatives.
Whanau has been nominated to represent Wellington City Council.
The group would be chaired by an independent expert, and supported by council chief executives and a joint project team.
It would not be a formal committee or have any decision-making rights.