Haskell has been in the role since early 2023 and will finish up on Friday.
This comes after an independent report into the entity that found staff took four months to tell the region's councils about a budget error leaving a $51 million bill over three years.
The water provider's board chair Nick Leggett said Haskell had helped grow the capital programme for water assets and was admired fronting media on difficult issues.
"In her time at Wellington Water, the capital programme delivering water assets for the region had grown from $60 million to $330m in the last financial year," he said.
The board would begin the process to appoint a new chief executive in the coming weeks.
"Working for Wellington Water has been a privilege," Haskell said in a statement.
"The team at Wellington Water, its supplier whānau and partners are incredibly smart, hard-working and dedicated to improving water outcomes for the region. I am proud to have been part of that."
Wellington has experienced a number of water problems in recent years, including sinkholes, burst pipes and leaks from ageing infrastructure. In February, Wellington Water said 44 percent of the capital's wastewater pipes and a quarter of the drinking water pipes were in 'poor' or 'very poor' condition, according to its latest modelling.
Wellington Water recently said an increased investment in leak repairs had contributed to a steady decline in water demand across Wellington, Porirua, Upper Hutt and Lower Hutt.
