The discoloured water coming out of taps across West Auckland is not a health risk, Watercare says.
Residents of Ranui, Kelston, Glen Eden and Henderson reported discoloured tap water on Friday evening.
Watercare said in a social media post it was flushing the network and "working as quickly as possible to get your water supply back to normal".
The council-controlled organisation thanked residents for their patience.
While the water looked "different" to normal, it was "still safe to drink", the post said.
"Our rigorous water sampling programme confirms that all the water we supply is compliant with the Drinking Water Standards of New Zealand."
The most common cause of discoloured tap water was minerals like iron and manganese that naturally built up in pipes, Watercare said.
"Some increased flow rate or a change in direction of the water flow can stir up these minerals and create a yellowish tinge in the water.
"There is also work underway in surrounding areas which can stir up sediment in the pipes."