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English club Harlequins confirmed yesterday the 31-year-old will join its ranks at the end of the season, following the Super Rugby season and a campaign with Wellington in the Mitre 10 Cup.
Highlanders general manager Roger Clark said Reddish, who played 15 games last season including the final, leaves with the franchise's best wishes.
"He's been outstanding,'' Clark said.
"He's added a lot to us in the time that he's been with us. We'll be sorry to see him go but we wish his family and him all the best.''
Reddish began his Super Rugby career with the Hurricanes in 2011 before Jamie Joseph added him to the roster for last year's campaign.
His involvement this year has been limited to four games.
Clark said the Highlanders would not have to look far for replacements in the second row.
"We've already got Josh Dickson signed for next year and Jackson Hemopo coming through, so locally we've got a couple of guys coming through,'' he said.
"They've been involved for a few years now so it's their time to stand up. Mark has done a wonderful job for us, but these young guys are getting a little bit older now and they've had a lot of experience over the last few years.
"That's what the programme is all about and you'd like to think they'll step up and do the job.''
Harlequins director of rugby John Kingston said in a statement on the club's website that Reddish would be a vital player in the squad when international commitments decimate the squad.
"His ability to play in both the second and back rows will provide additional flexibility,
"Mark's physical and uncompromising style will make him an invaluable part of the squad and we all look forward to welcoming him to Harlequins later in the year.''