Chief Ombudsman Peter Boshier is being forced to retire due to a 49-year-old law prohibiting him from keeping his post.
The Ombudsmen Act 1975, states that an Ombudsman "shall so resign his office on attaining the age of 72 years."
Boshier turns 72 on 16 March.
"The law is that I must resign from office upon reaching the age of 72," Boshier said in a statement.
"As such, I advised the Speaker of my resignation last week."
Boshier has been Chief Ombudsman since 2015.
"I continue to serve at the pleasure of Parliament. The Rt Hon Gerry Brownlee and the Officers of Parliament Committee will manage the process for the appointment of my successor," Boshier said.
The Chief Ombudsman is one of three officers of Parliament independent from the executive. The other two are the Parliamentary Commissioner for the Environment, and the Controller and Auditor-General.
Neither of those two roles have a mandatory retirement age.
Chief Ombudsmen are appointed on five-year terms, at the recommendation of the Officers of Parliament Committee, which is chaired by the Speaker.
The committee was aware of the section in the Act containing the age-limit when it recommended Boshier for re-appointment in April 2020.
Ahead of his re-appointment, Boshier told the committee - chaired by Trevor Mallard at the time - that his preference was a three-year term, ending around April 2023. He then wanted to discuss and engage with the committee about future plans at that time.
But Adrian Rurawhe, who was Speaker in April 2023, told RNZ the matter was not discussed by the Officers of Parliament Committee while he was Speaker.