Spark shares fell sharply as the market opened following the surprise news of managing director Simon Moutter's exit.
The stock slid 3.6 per cent to $3.64 in early trading. The broader NZX50 was 0.4 per cent.
Moutter will depart on July 1.
He will be replaced by customer director Jolie Hodson, who has been with the telco since 2013.
In her first interview, Hodson told the Herald she was committed to Spark Sport and other current initiatives.
Moutter has recently driven an "agile" restructure of Spark, plus an aggressive drive into sports content and streaming.
The Spark veteran had given the market no indication he was thinking about leaving - and his departure comes as his sports content strategy is still at a key stage, as Spark Sport launches and goes through some wobbles and faces criticism in some quarters that it is over-ambitious.
No reason for his stepping down was given this morning, beyond the old chestnut that he wanted to spend "some quality time with my family."
Moutter's total remuneration in 2018 was $2.82 million, plus a long-term incentive bonus of $1m outstanding.
Hodson was named Deloitte CFO of the Year in 2016. The same year, she told the Herald she wanted to break through the glass ceiling and become CEO one day.
Moutter served as chief operating officer in the Theresa Gattung era before leaving to become chief executive of Auckland Airport. He returned to Spark in 2012 to take the top job.
This morning, Spark chairwoman Justine Smyth said when Moutter became MD, he had done so in the expectation of a likely five-to-seven-year tenure.
• Jolie Hodson - Looking beyond the glass ceiling
When he became MD, Moutter was seen by some commentators as a fix-it technocrat who would focus on restructuring the telco's technology, organisation and staffing. He did that, but also initiated a bold move into content, first with Lightbox then Spark Sport.
Just how bold is still not known. Spark Sport's budget has never been made public.
• World Cup broadband jam: time will run out
Moutter has said on several occasions that Spark won't reveal the cost of acquiring and streaming sports content until its first financial results after the Rugby World Cup. Rugby World Cup.
"Simon leaves behind an extensive legacy of reform and redirection at the company formerly known as Telecom," tech commentator Paul Brislen says.
"He took an ex-state monopoly and did something quite unusual - turned it into a competitive retail player in the market. He deserves full credit for that."
Moutter's former number two at Spark, Jason Paris (recently made Vodafone NZ CEO) also paid tribute, tweeting:
Telecommunications Users Association head Craig Young said, "Simon has taken the hard choices and transformed Spark from a traditional telco when it split with Chorus and tried to position it for the future.
"It's good to see a quality internal candidate and another Kiwi step into the role in what remains our biggest retail service provider."
Spark's half-year profit fell 5.6 per cent to $153m in the six months to December 31 as it went without its usual profit-share dividend from the Southern Cross Cable - which faces new competition from the Hawaiki Australia-NZ-US cable backed by rich listers Malcolm Dick and Sir Eion Edgar.
Through his tenure, Spark's share price has climbed to $3.775 from $2.42 when he took over. The telco is also paying out annual dividends of 25 cents per share, compared to the 16 cents paid in his first year.
Matt Goodson, managing director of Salt Funds, said the news was a "slight surprise but not a shock".
"Moutter been well regarded by the market as having done an excellent job in holding the line with Spark and taking significant costs out of the business at a time when some of the legacy lines have been contracting," he said.
"He's done very well in playing the cards that he was dealt, as seen by the share price performance."
He said Hodson had clearly been "set" for the role and was well regarded by the market.
Sam Trethewey, portfolio manager at Milford Asset Management, said Hodson had done a "thorough apprenticeship" for the job.
He added, "The market had been questioning for some time as to when Simon would leave - he has been there for seven years.
"But I think it was something of a surprise - the timing of it - given what Simon has going on and the push into Spark sports."
Harbour Asset Management's Shane Solly noted, "Spark management has spent a lot of time recently emphasising there is no risk to spark's dividend."
Who is Jolie Hodson?
Hodson outlined her intentions to become a corporate chief executive as far back as 2016 in an interview with the Herald.
At the time, she was chief financial officer before moving up the ranks to become the chief executive of Spark Digital.
Hodson is an experienced business leader, having joined Spark in 2013 after a 12-year stint in Australia with liquor firm Lion, working her way up the finance division.
Before that, she spent eight years at Deloitte working as an auditor.
Her appointment to the Spark leadership role will make her one of only four women to be running one of the top 50 corporate entities in New Zealand.
The other women are A2 Milk's Jane Hrdlicka, Hallenstein-Glasson's Mary Devine and Chorus' Kate McKenzie.
She has long been outspoken about the importance of increasing gender diversity at a board level in New Zealand.
With reporting by Jamie Gray.