Mr Verbiest, who has been on the board since 2011, said he had best practice governance succession planning in mind when he decided to retire.
More than half of the current board, including Mr Verbiest, was appointed when Spark was demerged with Chorus in 2011.
"At some point, it will be appropriate for each of the long-serving directors to transition off the board.
"As a consequence, I do not believe it would be good governance to risk having several directors potentially retiring in short order.
"As the director with the longest association with Spark, I prefer to lead by example.
"In my view, the foundation for future success is solid and I feel the time to renew the chairmanship is now."
During his term as chairman, Spark had experienced 81% growth in market capitalisation, a total shareholder return of 145% and a return to revenue growth after 12 years of decline.
In a statement, Ms Smyth said it would be an honour to serve as chairwoman of Spark.
She paid tribute to the "enormous contribution" Mr Verbiest had made to Spark and, through the company, to New Zealand.
"He has always skilfully led discussion around the board table and sought to get the best possible outcome.
"As a result, he has set an outstanding governance example and helped drive the transformation of Spark as a business and as a driver of New Zealand’s economy."
