The UK-born chef was the toast of the London chef scene before his Kiwi wife, Angela, saw an advert on the internet for Soul, which was looking for a "world-class" chef.
Delighted to be invited to Auckland to try out for the role, he wowed his future colleagues with his cooking and promptly landed the job, moving to Auckland soon after.
"I have thoroughly enjoyed my time at Soul," Stewart told the Herald on Sunday.
But after nearly eight years, he said he was ready for something new.
"The challenge of heading up institutions like Euro and Jervois Steak House along with new favourites like The Crab Shack excites me immensely."
Sigley added his team were looking forward to Stewart's arrival.
"Gareth is a very talented chef," he said.
"He has some big shoes to fill with Simon's departure, but anyone who knows him, knows he is up for the challenge."
Stewart's next few months are busy.
Filming of the new TV2 season of My Kitchen Rules NZ is well under way with co-judge Ben Bayley of The Grove and Baduzzi. This season will see regional cook-offs.
A Soul spokeswoman confirmed it had appointed Philip Wilson as its new executive chef.
He had recently returned from a 10-year-stint in the UK and the spokeswoman said: "Watch this space."