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Poki (25), who will leave the Highlanders at the end of the season, joins Hosea Gear and Tamati Ellison in heading offshore.
Ellison is heading to Japan, to join the Ricoh club, while Gear is off to France, to play for Toulouse.
General manager Roger Clark said the Highlanders did not get a chance to make an offer to Poki as he signed with the Japanese club early in the season.
Poki has spent three years with the Highlanders, all under coach Jamie Joseph, and when he has been on the field he has played well.
But he has had injury issues, with rib problems hurting his performance this season. He did not make the three-week trip overseas last month and was hampered by a knee problem last year.
Clark said it was disappointing to lose Poki but Japanese rugby appeared to have plenty of cash and was a lot more aggressive in the market than it was a few years ago.
Poki has also played for Southland in the ITM Cup over the past couple of seasons.
Clark confirmed Highlanders loose forward Jake Paringatai was playing for Northland in the ITM Cup and had another year to run on his deal with the franchise.
Meanwhile, Clark said he was disappointed Otago players would not be available to play a Highlanders XV in an invitation game on June 21 at Forsyth Barr Stadium.
The Highlanders wanted to play one half against a Southland team and the other half against Otago, but Otago had declined the invitation, saying its players were committed to playing club rugby the next day.
Clark said it would be an opportunity for Otago players to test themselves at the next level and if they performed well, they would be in a good spot to win selection for the Highlanders next season.
The Chiefs had played a mixed team of Counties-Manukau and Bay of Plenty players during the June international window last year, Clark said, and that had helped promote many Counties-Manukau players into Super 15 spots this year.
Clark said it did not make sense to play earlier in the week as players and supporters were more attuned to play on a weekend date.
''What we found last year was the players have to get back into the momentum of playing games and that is done at the weekend,'' Clark said.
''And with this June window, we found many people lose track and it takes them a while to catch up. We have got the Crusaders game on that first Saturday and we want to make sure everyone is fully aware of it.''
Clark said Southland players were playing club rugby the following day and were eager to be involved.
About 12,000 tickets had already been sold for the game against the Crusaders on June 29, and the franchise had set aside a section in the south stand for Crusaders supporters.
Clark said the game was an opportunity to get a good crowd for the last home game of the season.