ESPNcricinfo understands that Conway is set to play for Joburg Super Kings in what will be his first appearance in South Africa since leaving the country and moving to New Zealand in 2017.

"Firstly, I'd like to thank New Zealand Cricket for their support through this process," Conway said. "The decision to move away from a central playing contract isn't one I've taken lightly, but I believe it is the best for me and my family at this present time.

"Playing for the Blackcaps is still the pinnacle for me and I'm hugely passionate about representing New Zealand and winning games of international cricket.

"I'm excited to be part of the upcoming Test squads for an important period in the ICC World Test Championship cycle and am looking forward to being involved in the ICC Champions Trophy in Pakistan next February if selected."

NZC chief executive Scott Weenink reiterated that casual contracts were only an option for a select number of the leading players but believed it was a prudent way to manage the evolving landscape of franchise cricket.

"We're delighted with Devon's decision to commit to the Blackcaps - he's a quality player who's made a strong contribution to the team over the past few years," he said. "In the current environment it's important to have flexibility in our system to navigate some of the challenges posed by franchise cricket - and this is another example of how we're working hard to retain our best players."

Conway and Allen join Williamson, Trent Boult, Lockie Ferguson and Adam Milne in moving away from central contracts although Williamson, who will also play the SA20 in January, has not ruled out taking one in the future.

Under the NZC system, to be eligible for a central contracts players need to make themselves available for the domestic Super Smash tournament when they aren't playing international cricket.