State Services Commissioner Mark Prebble denies that intense political pressure over public service employment scandals affected his decision to leave the job early.
Dr Prebble announced today he would leave at the end of June - a year before his contract expires.
He cited "minor health problems" as the reason, and said he had known for about two years that his tenure was coming to an end.
"For those who have been working closely with me, there's no surprise at all," he said.
"For ministers there's no surprise. I've been aware for a couple of years now that my ability to keep working at the rate I think this job needs is just getting more and more attenuated."
Dr Prebble said his health problems weren't life-threatening.
"It's basically that on too many days there have been flu-type symptoms and the like...it's just that for far too much of the year I haven't been well enough."
Dr Prebble was severely criticised by the National Party last year over the Madeleine Setchell employment scandal.
She lost her job in the Ministry for the Environment because of her relationship with a National Party press officer.
Then there was the appointment to the ministry's communications department of Clare Curran, who had Labour Party connections, and a subsequent scandal that involved an employee who claimed she left because of political interference.
Dr Prebble was accused of overseeing the politicisation of the public service, and there were calls for his resignation.
He said today he didn't resent the criticism because it was part of the job.
"It is in the nature of opposition that they tend to criticise," he said.
"It isn't personal. I've had criticism from Gerry Brownlee (National's state services spokesman) just as I've had criticism from other people in opposition, from a variety of parties."
Dr Prebble said claims that the Government had attempted to politicise the public service were wrong.
"We've done two inquiries, neither substantiated any such thing."
When he announced his decision, he said he had achieved many of the goals he set when he took on the job four years ago.
He said the most important step was establishing a Code of Conduct for the public service, which he published late last year.
Improved management systems and new ways to measure performance were also important achievements, he said.
Dr Prebble is ending a public service career that lasted more than 30 years, half of that as a senior manager in the Treasury, the Department of Prime Minister and Cabinet and the State Services Commission.
He said he had no plans for the future.
"I just don't have a plan. I can't say I'm a bit tired and then start looking for the next job...I'm just not interested in getting into talks with anybody for the next little while.
"I don't know what will happen after that."
Prime Minister Helen Clark said Dr Prebble told her in 2006 that he was not likely to serve a full term.
"I have very great respect for Mark and his abilities," she told reporters.
"I think it's important to put on the record he has had an extraordinary public service career...I've personally found him to be an exceptional public servant."
Miss Clark said Dr Prebble was not leaving under a cloud and there had been no pressure on him to leave the job.
Mr Brownlee said the National Party acknowledged Dr Prebble's long career as a devoted public servant.
"We wish him well as he deals with his health issues," Mr Brownlee said.
"The last few years have been extremely difficult for the public service as it has come under increasing pressure to run the Labour Party's political agenda."