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A statement from the company said that after nearly 50 years in broadcasting, Williams had decided to retire.
The 67-year-old was most recently hosting Magic Mornings on Magic Talk.
MediaWorks chief executive Cam Wallace acknowledged the contribution of Williams to the station.
"Peter has signalled for some time that he's wanted to finish up and thinks now is the right time," Wallace said.
"I'd like to acknowledge his significant contribution to Magic Mornings over the past three years - there have been plenty of engaging and lively discussions. MediaWorks wishes Peter all the best."
MediaWorks also welcomed Lloyd Burr, who recently returned to New Zealand after a three-and-half-year stint in London as Newshub's Europe correspondent.
Burr will join Magic Talk at the end of September and spoke about his enthusiasm for being back on Kiwi soil.
MediaWorks and Newshub further confirmed today that Ryan Bridge would be taking over the hosting responsibilities on the AM Show fulltime following the departure of Duncan Garner.
His shift to the morning created has created the gap for Burr to slip into the drive slot.
MediaWorks said that Leah Panapa would move into hosting 9am to midday on Magic Talk to replace Williams, while Danny Watson will continue to host afternoons of the station.
In addition to the changes among on-air talent, MediaWorks has also introduced a new executive recruit.
The company announced the appointment of Dallas Gurney as its director of news and talk.
Gurney most recently worked at media agency PHD, but had a long career in radio at Newstalk ZB and Radio Sport at NZME.
"Dallas has extensive experience in developing content strategy and a deep understanding of audiences, as well as a fantastic track record of leading teams and broadcasters to deliver best in market on-air digital content," Wallace said.
"We look forward to having Dallas join the team."
Gurney will start at the business later this month.