It's another blow for the TVNZ newsroom, which has lost newsgathering editor Brenda Griffiths and news manager Sarah Azam in the past few months.
Head of news and current affairs John Gillespie said: "As with any workplace we do have staff coming and going for their own reasons. As an organisation TVNZ takes gender diversity very seriously, both internally and in the work we produce."
McCormack is leaving on a high with Sunday rating its socks off. This past week the current affairs programme reached ratings of 681,860.
It's a long way from where she first started at TVNZ aged 18, sweeping the dockway for little pay. Paul Holmes would come out for a ciggie and a chat, and they developed a lifelong friendship.
McCormack, who is married to TVNZ journalist Mark Crysell, with whom she has a young daughter, Edie, quickly rose up the ranks and has been a valued member of the TVNZ news management.
Her friend and rival Pip Keane, executive producer of Campbell Live, is being touted as a possible replacement. But I'm told it will be a hotly contested job.
Last week, former Justice Minister Judith Collins told TVNZ bosses she wants its news bulletins to take women more seriously.
In the annual review of TVNZ by Parliament's commerce committee, Collins and MPs Gareth Hughes and Clare Curran called for the national broadcaster to address its attitude towards gender equality in its news coverage.
In the report the committee said, "We would like TVNZ's news items to feature less categorising of women."
Collins told the Herald on Sunday: "Too often, women are portrayed in the news as victims of circumstance or their roles in life tend to be stereotyped.
"Strong women are referred to as being strident or aggressive whereas strong men are simply referred to as being strong."
Collins said she believed TVNZ in general did a good job but expected changes to bring more gender equality to the news.
Ex MediaWorks boss joins TVNZ board
TVNZ is making great strides in gender equality at board level.
Sussan Turner, former chief executive of MediaWorks, has been appointed as a director of TVNZ and joins the board for three years. Therese Walsh has also been appointed deputy chairwoman, which means the TVNZ board now has a female majority.
"As a former group CEO of MediaWorks, Ms Turner has extensive television and media knowledge and experience, and will bring real depth and substance to the role," Broadcasting Minister Amy Adams said in a statement.
"Ms Walsh has been a director of TVNZ since 2012 and has made a considerable contribution since her appointment.
"She will provide strong leadership support to the board in her new position as deputy chair."